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Keywords:

  • additives;
  • benzenetrisamides;
  • nucleation;
  • nucleating agents;
  • poly(propylene);
  • self-assembly;
  • supramolecular

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Experimental Section
  5. 3. Results and Discussion
  6. 4. Conclusions
  7. Acknowledgements

We present a family of 2,4,6-trimethyl-1,3,5-benzenetrisamides for ultra-efficient nucleation of isotactic polypropylene (i-PP). On the basis of a new symmetrically substituted core, in a series of compounds, the chemical structure of peripheral substituents is systematically varied, introducing branched aliphatic, aromatic, and cycloaliphatic moieties. Some of these compounds are found to promote nucleation of the α-phase in i-PP at concentrations as low as 0.00003 wt% only, while concomitantly featuring outstanding thermal properties.


1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Experimental Section
  5. 3. Results and Discussion
  6. 4. Conclusions
  7. Acknowledgements

Crystallization of polymers has attracted scientific and industrial attention for many decades, due to its intriguing molecular process and commercial interests. Crystallization of a macromolecule of sufficient chain regularity occurs upon cooling from its melt or solution below the corresponding melting or dissolution temperature at a certain supercooling. That latter driving force is of major significance as it controls production cycle rates and energy consumption involved in the creation of polymer artifacts made with crystallizable macromolecular species. In order to reduce that supercooling, often additives are employed that raise the crystallization temperature of the polymer. Additional benefits from the addition of such so-called nucleating agents are reducing the size of spherulitic entities into which many polymers crystallize, which improves mechanical, surface, and optical properties.1

One important polymer in need of enhanced nucleation is the slowly self-nucleating isotactic polypropylene (i-PP). Classical nucleating agents for i-PP are, among others, inorganic or organic salts,2 such as sodium benzoate, sodium 2,2′-methylene-bis-(4,6-di-tert-butylphenyl)phosphate,3, 4 and disodium bicyclo[2.2.1]heptane-2,3-dicarboxylate.5, 6 One drawback of such additives is the difficulty of achieving homogeneous dispersion in the polymer due to their insolubility. A group of (organic) nucleating agents for i-PP that avoid this issue, and indeed supramolecular nucleating agents are soluble in the molten polymer at elevated temperatures and below a certain concentration. The most prominent class is based on carbohydrates, which include 1,3:2,4-bis(p-methylbenzylidene) sorbitol, 1,3:2,4-bis(p-ethylbenzylidene)sorbitol 1,3:2,4-bis(3,4-dimethylbenzylidene)sorbitol, and 1,2,3-trideoxy-4,6:5,7-bis-O-[(4-propylphenyl)methylene]-nonitol with which outstanding dispersion can be achieved indeed.7 However, due to their carbohydrate chemical nature, important disadvantages limit their use,8 such as thermal blooming, sublimation and decomposition of the additive resulting in migration,9 bubble formation, plate out,10 and discolored and odorous products.11, 12 In recent years, a new class of supramolecular nucleating agents based on 1,3,5-benzenetrisamides was developed that circumvents the above disadvantages. Many compounds of this family were found to raise the crystallization temperatures of i-PP and some of them promoted so-called clarifying, that is, drastic reduction of haze and increasing clarity, at concentrations as low as 0.02 wt%,13 which is one order of magnitude lower than for rosin-type14 and sorbitol clarifiers.15 In addition of being soluble in the polymer melt permitting ultra-fine dispersion,15 1,3,5-benzenetrisamides exhibit outstanding thermal stability.13, 16

In extension of investigations of this family of compounds,13, 17 we report here on the synthesis and characterization of 2,4,6-trimethyl-1,3,5-benzenetrisamides 111 as nucleating agents for i-PP (Scheme 1). In this series of nucleating agents for the first time, the central core is symmetrically substituted with three methyl groups with the intention to influence the supramolecular packing. This was stimulated by publications by Matsunaga et al.18, 19 on discotic liquid crystals. The introduction of a core substitution with three methyl substitutents increases the melting temperatures substantially of otherwise identical 1,3,5-benzenetrisamides.18, 19 The higher melting temperatures should lead to improved temperature stable additives. In addition to the trimethyl core substitution, the chemical structure of the peripheral groups was systematically varied by introducing branched aliphatic, aromatic, and cycloaliphatic substituents since small/minor changes in the peripheral substituents have a strong influence on the nucleation efficiency.13, 17 These compounds were explored with respect to their thermal stability and nucleation efficiency in i-PP.

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Scheme 1. Chemical structures of 2,4,6-trimethyl-1,3,5-benzenetrisamides 1–11. The peripheral substituents R are drawn with their left bond to the carbonyl C atoms.

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2. Experimental Section

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Experimental Section
  5. 3. Results and Discussion
  6. 4. Conclusions
  7. Acknowledgements

2.1. Materials

1,3,5-Trisamino-2,4,6-trimethylbenzene was provided by Ciba Specialty Chemicals, now BASF SE, Basel. Acid chlorides and amines were obtained from ABCR, Acros, Aldrich, and Fluka and used as received. Solvents were purified and dried according to standard procedures. 1H NMR spectra were recorded at room temperature on a Bruker AC 250 instrument. Mass spectra were recorded on a VARIAN MAT 7 (direct probe inlet, electron impact ionization) at the Central Analytic Laboratories at the University of Bayreuth.

2.2. Synthesis

2.2.1. General Procedure

Under inert atmosphere and dry conditions, the corresponding acid chloride is added at 0 °C to a solution consisting of N-methylpyrrolidone (NMP)/pyridine or NMP/triethylamine, LiCl, and 1,3,5-trisamino-2,4,6-trimethylbenzene. The reaction mixture is heated to 80 °C and kept at this temperature for two additional hours, unless indicated differently. After cooling, the reaction mixture was precipitated into fivefold excess ice water. The precipitate is filtered and dried under vacuum and recrystallized from solvents as specified.

2.2.2. Individual Compunds

1,3,5-Tris(2-methylpropionylamino)-2,4,6-trimethylbenzene (1): 6.61 g (62.04 mmol) 2-methylpropionyl chloride, 2.48 g (15.00 mmol) 1,3,5-trisamino-2,4,6-trimethylbenzene, 100 mL NMP, 25 mL triethylamine, 0.1 g LiCl. Reaction conditions: 48 h, 70 °C. Purification: Soxleth extraction with methanol. Yield: 4.19 g (11.15 mmol), 74%. Characterization: 1H NMR (CDCl3/CF3COOD): δ = 1.35 (d, J = 7.0 Hz, 18H); 2.10 (d, J = 5.5 Hz, 9H); 2.84 (sept., J = 7.0 Hz, 3H). MS (70 eV), m/z (%): 375 (M+, 87); 332 (38); 305 (99); 289 (12); 262 (32); 253 (41); 218 (13); 192 (11); 175 (22); 164 (26); 148 (9); 137 (9); 71 (19); 43 (100); 41 (17). Thermal properties: Tm: -, Ts: 464 °C, T-10 wt%: 418 °C.

1,3,5-Tris(2,2-dimethylpropionylamino)-2,4,6-trimethylbenzene (2): 6.03 g (50.00 mmol) 2,2-dimethylpropionyl chloride, 2.48 g (15.00 mmol) 1,3,5-trisamino-2,4,6-trimethylbenzene, 150 mL NMP, 25 mL pyridine, 0.1 g LiCl. Reaction conditions: 12 h, 60 °C. Purification: Recrystallization from acetone. Yield: 3.92 g (9.39 mmol), 63%. Characterization: 1H NMR (DMSO-d6): δ = 1.23 (s, 27H); 1.89 (s, 9H); 8.89 (s, 3H). MS (70 eV), m/z (%): 417 (M+, 71); 402 (19); 374 (28); 362 (13); 346 (8); 319 (100); 276 (40); 263 (35); 246 (64); 221 (28); 207 (17); 190 (15); 178 (13); 165 (24); 148 (17); 123 (17); 95 (11); 81 (11); 71 (9); 43 (52). Thermal properties: Tm: 289 °C, T-10 wt%: 368 °C.

1,3,5-Tris(2-ethylbutyrylamino)-2,4,6-trimethylbenzene (3): 6.73 g (50.00 mmol) 2-ethyl-butyryl chloride, 2.48 g (15.00 mmol) 1,3,5-trisamino-2,4,6-trimethylbenzene, 100 mL NMP, 25 ml triethylamine, 0.1 g LiCl. Reaction conditions: 24 h, 60 °C. Purification: Recrystallization from methanol. Yield: 4.60 g (10.01 mmol), 66%. Characterization: 1H NMR (CDCl3/CF3COOD): δ = 1.05 (t, J = 7.4 Hz, 18H); 1.68–1.79 (m, 12H); 2.17 (s, 9H); 2.45 (quint., 7.1 Hz, 3H). MS (70 eV), m/z (%): 459 (M+, 26); 388 (12); 361 (100); 290 (9); 263 (30); 164 (10); 71 (22); 43 (22). Thermal properties: Tm: -,Ts: 468 °C, T-10 wt%: 430 °C.

1,3,5-Tris(3,3-dimethylbutyrylamino)-2,4,6-trimethylbenzene (4): 7.27 g (54.00 mmol) 3,3-dimethylbutyryl chloride, 2.48 g (15.00 mmol) 1,3,5-trisamino-2,4,6-trimethylbenzene, 150 mL NMP, 30 mL triethylamine, 0.4 g LiCl. Reaction conditions: 12 h, 60 °C. Purification: Recrystallization from DMF. Yield: 4.30 g (9.35 mmol), 62%. Characterization: 1H NMR (DMSO-d6): δ = 1.06 (s, 27H); 1.97(s, 9H); 2.21 (s, 6H) 9.14 (s, 3H). MS (70 eV), m/z (%): 459 (M+, 22); 444 (17); 361 (100); 263 (48); 164 (17); 57 (19). Thermal properties: Tm: -, Ts: 391°C, T-10 wt%: 383 °C.

1,3,5-Tris(benzoylamino)-2,4,6-trimethylbenzene (5): 5.06 g (36.00 mmol) benzoyl chloride, 1.77 g (10.71 mmol) 1,3,5-trisamino-2,4,6-trimethylbenzene, 100 mL NMP, 20 mL triethylamine, 0.3 g LiCl, Reaction conditions: 12 h, 60 °C. Purification: Precipiation from DMF in acetone. Yield: 3.46 g (7.24 mmol), 72%. Characterization: 1H NMR (DMSO-d6): δ = 2.10 (s, 9H); 7.50–7.63 (m, 9H); 8.02–8.05 (m, 6H) 9.95 (s, 3H). MS (70 eV), m/z (%): 477 (M+, 22); 459 (12); 372 (26); 105 (100); 77 (39). Thermal properties: Tm: 354 °C, T-10 wt%: 446 °C.

1,3,5-Tris(4-methylbenzoylamino)-2,4,6-trimethylbenzene (6): 5.56 g (35.96 mmol) 4-methylbenzoyl chloride, 1.65 g (10.00 mmol) 1,3,5-trisamino-2,4,6-trimethylbenzene, 100 mL NMP, 20 mL triethylamine, 0.3 g LiCl. Reaction conditions: 12 h, 60 °C. Purification: Recrystallization two times from chloroform. Yield: 3.83 g (7.37 mmol), 73%. Characterization: 1H NMR (DMSO-d6): δ = 2.07 (s, 9H); 2.38 (s, 9H); 7.33 (d, J = 8.1 Hz, 6H) 7.94 (d, J = 8.1 Hz, 6H); 9.84 (s, 3H). MS (70 eV), m/z (%): 519 (M+, 39); 501 (14); 400 (26); 119 (100); 91 (29). Thermal properties: Tm: 377 °C, T-10 wt%: 418 °C.

1,3,5-Tris(3,4-dimethylbenzoylamino)-2,4,6-trimethylbenzene (7): 9.10 g (53.96 mmol) 3,4-dimethylbenzoyl chloride, 2.48 g (15.00 mmol) 1,3,5-trisamino-2,4,6-trimethylbenzene, 150 mL NMP, 30 mL triethylamine, 0.4 g LiCl. Reaction conditions: 12 h, 60 °C. Purification: Recrystallization from methanol. Yield: 6.62 g (11.79 mmol), 79%. Characterization: 1H NMR (DMSO-d6): δ = 2.07 (s, 9H); 2.30 (s, 18H); 7.28 (d, J = 7.9 Hz, 3H) 7.77 (m, 6H); 9.80 (s, 3H). MS (70 eV), m/z (%): 561 (M+, 19); 428 (10); 133 (100); 105 (26); 89 (8). Thermal properties: Tm: -, Ts: 443°C, T-10 wt%: 439 °C.

1,3,5-Tris(3,5-dimethylbenzoylamino)-2,4,6-trimethylbenzene (8): 9.10 g (53.96 mmol) 3,5-dimethylbenzoyl chloride, 2.48 g (15.00 mmol) 1,3,5-trisamino-2,4,6-trimethylbenzene, 150 ml NMP, 30 ml triethylamine, 0.4 g LiCl. Reaction conditions: 12 h, 60 °C. Purification: Recrystallization from DMF. Yield: 5.18 g (9.22 mmol), 61% Characterization: 1H NMR (DMSO-d6): δ = 2.06 (s, 9H); 2.35 (s, 18H); 7.22 (s, 3H); 7.64 (s, 6H); 9.82 (s, 3H). MS (70 eV), m/z (%): 561 (M+, 25); 428 (21); 133 (100); 105 (40); 79 (9). Thermal properties: Tm: -, Ts: 448 °C, T-10 wt%: 453 °C.

1,3,5-Tris(cyclopentylcarbonylamino)-2,4,6-trimethylbenzene (9): 7.16 g (54.00 mmol) cyclopentylcarbonyl chloride, 2.48 g (15.00 mmol) 1,3,5-trisamino-2,4,6-trimethylbenzene, 150 mL NMP, 30 mL pyridine, 0.4 g LiCl, Reaction conditions: 24 h, 60 °C. Purification: Recrystallization from DMF. Yield: 5.20 g (11.46 mmol), 76%. Characterization: 1H NMR (DMSO-d6): δ = 1.57–1.99 (m, 33H); 2.73–2.89 (m, 3H); 9.16 (s, 3H). MS (70 eV), m/z (%): 453 (M+, 31); 384 (17); 357 (100); 288 (9); 261 (25); 164 (11); 69 (26). Thermal properties: Tm: -, Ts: 461 °C, T-10 wt%: 429 °C.

1,3,5-Tris(cyclohexylcarbonylamino)-2,4,6-trimethylbenzene (10): 8.24 g (56.20 mmol) cyclohexylcarbonyl chloride, 2.58 g (15.60 mmol) 1,3,5-trisamino-2,4,6-trimethylbenzene, 150 mL NMP, 30 mL pyridine, 0.4 g LiCl, Reaction conditions: 12 h, 60 °C. Purification: Recrystallization from DMF. Yield: 6.01 g (5.87 mmol), 81%. Characterization: 1H NMR (DMSO-d6): δ = 1.15-1.97 (m, 39H); 2.32-2.41 (m, 3H); 9.09 (s 3H). MS (70 eV), m/z (%): 495 (M+ 38); 412 (22); 385 (100); 302 (11); 275 (26); 164 (13); 83 (31); 55 (24). Thermal properties: Tm: -,Ts: 461 °C, T-10 wt%: 438 °C.

1,3,5-Tris(1-adamantanecarbonylamino)-2,4,6-trimethylbenzene (11): 7.15 g (36.00 mmol) 1-adamantanecarbonyl chloride, 1.65 g (10.00 mmol) 1,3,5-trisamino-2,4,6-trimethylbenzene, 100 mL NMP, 20 mL triethylamine, 0.3 g LiCl, Reaction conditions: 12 h, 80 °C. Purification: Recrystallization two times from chloroform. Yield: 0.50 g (0.77 mmol), 8%. Characterization: 1H NMR (DMSO-d6): δ = 1.70 (m, 54H); 8.79 (s, 3H). MS (70 eV), m/z (%): 651 (M+, 9); 623 (9); 566(11); 488(11); 135(100); 108(11); 93 (20); 79 (16). Thermal properties: Tm: -, Ts: 436 °C, T-10 wt%: 440 °C.

2.3. Processing

The i-PP homopolymer grade used was Profax PH350 (Montell) with a number average molar mass of ≈6 × 104 g mol−1 and a weight average molar mass of ≈4 × 105 g mol−1. The grade contains 0.05 wt% of the antioxidant Irganox 1010, 0.10 wt% Irgafos 168, and 0.1 wt% calcium stearate. Prior to use, the polymer pellets were pulverized in an Ultra Centrifugal Mill (Retsch ZM100) with liquid nitrogen cooling and a 1000 μm sieve.

2.3.1. Compounding

The pulverized i-PP were dry blended with the respective powdered additive and compounded in a laboratory corotating twin-screw mixer (Technical University Eindhoven, the Netherlands) at 240 °C under nitrogen atmosphere. Series of i-PP with different additive concentrations were prepared commencing with an i-PP masterbatch (5.6 g) comprising 0.15 wt% additive, which was subsequently diluted with neat i-PP, hereafter referred as a “dilution series.” For each concentration, 3.0 g of the compounded mixture was discharged. To the remaining 2.6 g, an amount of 3.0 g of neat i–PP was added, first resulting in a mixture comprising 0.07 wt% of the additive. After again discharging 3.0 g of this mixture, again 3.0 g neat i-PP was added resulting in a mixture containing 0.032 wt% additive. By repeating this procedure, blends of i-PP and additive concentrations ranging from 0.15 to 0.00003 wt% were prepared. In each dilution, the compounding period was 4 min. Neat i-PP was treated in the same way to generate blank control extrudates with the same multiple treatments. To ensure the polymers stability and eliminate influences of the high number of dilution runs, we conducted DSC-measurements of neat i-PP, which was processed for 12 dilution runs as described above. It was found that the polymer crystallization temperature deviation is ±0.5 °C.

2.4. Characterization Methods

2.4.1. Optical Microscopy

Optical micrographs were taken between crossed polarizers with Nikon ACT-1 software using a digital camera (Nikon, DMX1200) attached to an optical microscope (Nikon, DIAPHOT 300) equipped with a hot stage (Mettler, FP82HT).

2.4.2. Thermal Analysis

Thermogravimetric analysis (TGA) of benzenetrisamides was conducted using a Netzsch STA 409 instrument under nitrogen atmosphere (20 mL min−1) at a heating rate of 10 K min−1. The temperatures at which a weight loss of 10 wt% occurred are reported (T-10 wt%). The sublimation temperature (Ts) was determined at the peak temperature of the first deviation of the weight loss curves. Compound with at least 95% weight loss are considered to sublime. Melting temperatures (Tm) of benzenetrisamides were simultaneously detected with differential thermal analysis (DTA) and reported are the peak maximum of the endothermic transition. Differential scanning calorimetry (DSC) was performed using a PerkinElmer (DSC, Diamond) apparatus with samples of 5–10 mg at standard heating and cooling rates of 10 K min−1 under nitrogen (20 mL min−1). Between 50 and 230 °C, two heating and cooling scans were recorded. Prior to cooling, the samples were held for 5 min at 230 °C. The temperature of the polymer crystallization (Tc,p) is reported as the minimum of the exothermic peak in the first cooling. To determine the nucleation efficiency (NE), self-seeding experiments as established by Lotz and co-workers20 were performed. This involves the partial melting of the polymer at an arbitrary temperature Ts between the maximum of the endothermic melting peak and its offset, where upon cooling, the remaining crystal fragments act as perfect nuclei for the crystallization of i-PP and increase the crystallization temperature. The value for the highest achievable polymer crystallization temperature of i-PP (Tc,p max = 140.8 °C) is obtained at the exothermic peak minimum upon cooling from Ts = 162.9 °C. The increased polymer crystallization temperature after the addition of a nucleating agent is defined as Tc,p nucl. The crystallization temperature of neat i-PP Profax PH350 was determined to Tc,p neat = 112.7 °C. Now the NE is calculated by

  • equation image((1))
2.4.3. Wide-Angle X-ray Scattering

To evaluate the β-content in injection-molded i-PP plaques of the various additives (0.15 wt%), wide-angle X-ray scattering (WAXS) patterns were recorded in transmission with a Bruker D8 Advance X-ray diffractometer. The wavelength used was Cu-Kα (λ = 1.54 Å) and spectra were recorded in the 2Θ range of 8–30° (step size 0.025°). From these X-ray data, k-values were calculated according to standard procedures given in the literature.13 The k-value thus is proportional to the content of the β-crystal modification in the sample.

2.4.4. Scanning Electron Microscopy

Sample preparation: 2,2,4,4,6,8,8-heptamethylnonane (HMN) purchased from Aldrich was freshly distilled under reduced pressure prior to use. Compound 9 was dissolved in this fluid for 10 min at 240°C in a high pressure DSC-pan at a concentration of 0.007 wt% and recrystallized at a cooling rate of 60 K min−1. About 10 μL of the obtained suspension was transferred to an Al sample carrier for SEM and the HMN was evaporated under vacuum at room temperature for 2 d. The samples were sputtered with platinum (0.8 nm) by a Cressington Sputter Coater 208HR. Scanning electron micrographs were recorded using Zeiss 1530 FESEM.

3. Results and Discussion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Experimental Section
  5. 3. Results and Discussion
  6. 4. Conclusions
  7. Acknowledgements

Thermal properties of the 2,4,6-trimethyl-1,3,5-triaminobenzenes 111 were examined with combined TGA and DTA under nitrogen atmosphere at a heating rate of 10 °C min−1. The results are summarized in Table 1.

Table 1. Thermal properties, such as melting (m, Tm) and sublimation temperature (s, Ts), and 10% weight loss (T-10 wt%), of 2,4,6-trimethyl-1,3,5-benzenetrisamides 111 and their performance as additives in isotactic polypropylene (polymer crystallization temperature, Tc,p, and nucleation efficiency, NE) at a concentration of 0.15 wt%. The structure of the substituent R is drawn with its left bond to the carbonyl C atom.
No.Additive/SubstituentRTm,Ts [°C]T10 wt% [°C]Tc,p [°C]NE [%]
1i-Propyl
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464 s418128.255.2
2tert-Butyl
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289 m368113.83.9
31-Ethylpropyl
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468 s430124.140.6
42,2-Dimethylpropyl
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391 s383113.01.1
5Phenyl
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354 m446126.348.4
64-Methylphenyl
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377 m418125.545.6
73,4-Dimethylphenyl
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443 s439118.420.3
83,5-Dimethylphenyl
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448 s453120.828.8
9Cyclopentyl
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461 s429128.656.6
10Cyclohexyl
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461 s438127.853.7
111-Adamantyl
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436 s440114.25.3
 Polypropylene neat165 m112.7

All 2,4,6-trimethyl-1,3,5-benzenetrisamides were found to be highly thermally stable, most of them to temperatures above 400 °C. These thermal characteristics can be categorized in two groups. The first group of compounds features a simple melting transition followed by evaporation from the liquid phase. For example, compound 2 has a melting transition Tm at 289 °C. The 10% weight loss (T-10 wt%) is detected at 368 °C and followed by 100% weight loss. The same behavior was found for compound 5 and 6. It is interesting to point out that the melting temperature of compound 2 (289 °C) with the t-butyl substituent is significantly lower than for 5 (354 °C) and 6 (377 °C) with aromatic substituents, thus indicating an additional π–π interaction of these groups. The values of T-10 wt% in all cases are well above 360 °C. For the second group of compounds (1, 3, 4, 7, 8, 9, 10, 11) sublimation is detected, all above 390 °C. Exemplary, the thermal behavior of compound 9 is shown in Figure 1, with the sublimation peak at 461 °C.

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Figure 1. Thermogravimetric (solid black line) and differential thermal analysis (dashed red line) of the cyclopentyl substituted 2,4,6-trimethyl-1,3,5-benzenetrisamide 9 (nitrogen atmosphere, heating rate of 10 K min−1).

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Compared with the clearing temperatures reported by Harada and Matsunaga19 for compounds with linear n-alkyl chains from n = 5 to 15, the compounds presented here feature much higher melting or sublimation temperatures and do not exhibit liquid crystalline mesophases.

3.1. Crystallization and Nucleation

As typical example, compound 9 was recrystallized as described in the “Section 2” from the high-boiling hydrocarbon 2,2,4,4,6,8,8-heptamethylnonane (HMN; bp: 240 °C). A similar approach had been employed in the past by Binsbergen using squalane C30H62 to mimic the nonpolar environment of i-PP.21 In contradiction to solvent used by him, which has to be removed by extraction, HMN can be completely removed under vacuum at room temperature. A scanning electron micrograph of the thus prepared crystalline entities of 9 is presented in Figure 2.

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Figure 2. Scanning electron micrograph of whiskers of the cyclopentyl substituted 2,4,6-trimethyl-1,3,5-benzenetrisamide 9 prepared by recrystallization from 2,2,4,4,6,8,8-heptamethylnonane (HMN) at a concentration of 0.007 wt% at cooling rate of 60 K min−1.

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As demonstrated by this image, this compound features a fibrillar, whisker-like crystals of diameters in the 50–200 nanometer range, indicative of preferential 1-dimensional columnar molecular aggregation.22 A similar, a whisker-like needle prepared at higher concentrations was placed onto a compression-molded i-PP film and examined under the polarized light microscope. In Figure 3 are shown micrographs of this fibrillar crystal of 9 surrounded by the supercooled i-PP melt at a temperature of 136 °C (left) and at 131 °C (right). The latter micrograph clearly reveals its nucleation ability to induced oriented growth of i-PP featuring as the orange area around the bluish whisker.

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Figure 3. Polarized optical micrographs (taken with a λ/4 wave plate) of a needle of the cyclopentyl substituted 2,4,6-trimethyl-1,3,5-benzenetrisamide 9 in a supercooled melt of i-PP at 136 °C (left). Upon cooling at 5 K min−1 to 131 °C nucleation and trans-crystallization of i-PP occurs on the additive surface. Note the full absence of any crystal of i-PP formation in the surrounding supercooled melt.

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3.2. i-PP-Additive Compounds

In a first set, mixtures were produced with i-PP comprising standard additive concentration of 0.15 wt% to quantitatively evaluate their nucleating potential as deduced from the crystallization temperature (Tc,p) of the polymer (Table 1). Remarkably, crystallization temperatures of i-PP vary from almost that of the neat polymer (2, 4, and 11) to values up around 128 °C achieved with 1, 9, and 10. To determine the NE, self-seeding experiments were performed.20 These experiments revealed that the highest achievable polymer crystallization temperature Tc,p max for this i-PP grade is at 140.8 °C. The crystallization temperature of neat i-PP Profax PH350 was determined to Tc,p neat = 112.7 °C. With these values, the NE can be calculated. Most efficient are compound 1 (R: i-propyl), 9 (R: cyclopentyl), and 10 (R: cyclohexyl), with nucleation efficiencies in the range of 53% to 57% (Table 1).

In addition, wide-angle X-ray diffraction experiments were performed to reveal to which extent the α-phase or the β-phase of i-PP is induced by these nucleating agents. The ability to promote nucleation of the α-phase of i-PP was determined and is expressed as the k-value, which is proportional to the content of the β-modification in the sample.13 All 2,4,6-trimethyl-1,3,5-benzenetrisamides/i-PP compounds—with exception of derivative 1 (R: i-propyl, k = 0.31)—form predominantly the α modification of i-PP This is indicated by the low k values between 0.10 and 0.17.

To illustrate how complicated it is to establish structure–property relations in the matter of nucleating agents for i-PP, an interesting comparison with the crystallization temperatures (Tc,p) of previously published17 i-PP compounds at the same concentration with analogues additives without the trimethyl substitution at the core can be made. For instance, additive 1 (R: i-propyl) exhibits a Tc,p of 128.2 °C, whereas for the same R without the trimethyl substitution at the core, a much lower Tc,p of 117.5 °C was observed. However, for R: tert-butyl, the Tc,p values show the opposite. Additive 2 of this study has a low Tc,p of 113.8 °C, whereas the corresponding additive without core substitution has a high Tc,p of 124.8 °C. For the additives 9 (R = cyclopentyl) and 10 (R = cyclohexyl), the Tc,p temperatures are with 128.6 °C and 127.8 °C very high whereas the corresponding additives without core substitution show no significant nucleation effect (Tc,p = 114.5 °C and 113.9 °C). This demonstrates that small and subtle changes at the core and of the substituents alter the molecular packing and supramolecular structure of the nuclei and hence varies the epitaxial matching with i-PP.

3.3. Additive Concentration

It is well established that nucleation, and more pronounced optical properties, depends on the phase behavior—that is, temperature/composition—of polymer-additive systems.7, 13, 17 Therefore, the influence of the additive content on their performance in compounds with i-PP was examined for the substances that featured the highest polymer crystallization temperature in the above studies, that is, 1, 9, and 10. Polymer-additive compounds with compositions ranging from 0.15 to 0.00003 wt% of the nucleating agent were prepared by consecutive dilution, as described in the “Section 2.” The crystallization temperatures of i-PP are plotted against concentration of the different additives as shown in Figure 4. Very low concentrations always raise the question of reproducibility.

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Figure 4. Crystallization temperatures (Tc,p) of i-PP with the isopropyl substituted 1, cyclopentyl substituted 9, and cyclohexyl substituted 2,4,6-trimethyl-1,3,5-benzenetrisamide 10 as function of their concentration. The solid symbols are the data of a complete dilution series. The open symbols are data of a second dilution series to demonstrate reproducibility. Compound 9 was individually compounded at a concentration of 0.015 wt%, for which Tc,p is indicated with a red diamond. The red arrow indicates the crystallization temperature of neat i-PP processed under identical conditions.

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To demonstrate the powerful tool of dilution experiments to realize very small concentrations, Figure 4 also depicts the data of a second series (open symbols), which show a excellent agreement with the first series (solid symbols). Additionally, compound 9 was compounded at a single concentration of 0.015 wt% (red symbol in the middle plot of Figure 4 for 9), and also this Tc,p data point shows little deviation from the other two dilution experiments. Most surprisingly, in all cases, only a modest decrease of Tc,p with decreasing additive concentration over almost five orders of magnitude is observed—at weight fractions of the nucleating agents as low as 3 × 10−5 wt%. In Figure 5, values of Tc,p versus additive content are plotted for compound 9 and sodium 2,2′–methylene-bis-(4,6-di–tert-butylphenyl)phosphate (NA-11), an inorganic commercial nucleation additive for i-PP.

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Figure 5. Crystallization temperatures (Tc,p) of i-PP versus additive content of 2,4,6-trimethyl-1,3,5-benzenetrisamide 9 (R: cyclopentyl, solid black dots), and NA-11 (open red triangles) determined in course of this work. Values of NA-11 reported in literature are indicated by solid red triangles. The red arrow indicates the crystallization temperature of neat i-PP processed under identical conditions.

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For NA-11, data of our own dilution series and data from literature are plotted. The change of the Tc,p for the presented additive 9 is very low even at extremely low concentrations (3 × 10−5 wt%), however, NA-11 looses its performance already around 0.1 wt%.

4. Conclusions

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Experimental Section
  5. 3. Results and Discussion
  6. 4. Conclusions
  7. Acknowledgements

The above results demonstrate that particular members of the new family of 2,4,6-trimethyl-1,3,5-benzentrisamides offer a set of uniquely ultra-efficient nucleating agents for isotactic polypropylene at very low concentrations. This is especially illustrated by the data presented in Figure 4 and Figure 5, in which it is shown that compounds 1, 9, and 10 remain efficient nucleating agents at contents as low as 0.00003 wt%, while the much-touted compound NA–11 fails to perform already at concentrations as high as ≈0.1 wt%. Very low additive concentrations as presented here are important for medical and food packaging applications, and the reduction of additive load levels while maintaining the overall performance is a shared aim in industry and academia.

Acknowledgements

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Experimental Section
  5. 3. Results and Discussion
  6. 4. Conclusions
  7. Acknowledgements

We gratefully acknowledge the DFG (SFB 840, Project B4) for financial support, Sandra Ganzleben and Doris Hanft for their assistance in synthesizing the additives and Jutta Failner for preparing the additive mixtures. 1,3,5-Trisamino-2,4,6-trimethylbenzene was provided by Ciba Specialty Chemicals, now BASF S.E., Basel, Switzerland.