Influence of Tacticity on Structural Ordering of Isotactic Polypropylene under Annealing

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1 nadian Chemical Transactions Research Article DOI: /canchemtrans Influence of Tacticity on Structural Ordering of Isotactic Polypropylene under Annealing Al Mamun 1* and Mohammad A K Khan 2 1 Department of Chemistry, University of Montreal, Montreal, Quebec, nada H3C3J7 2 Department of Chemistry and Chemical Biology, McMaster University, Hamilton, Ontario, nada L8S 4M1 * Corresponding Author, amamun@fsu.edu Phone: , Fax: Received: November17, 2013 Revised: December 9, 2013 Accepted: December 11, 2013 Published: December 12, 2013 Abstract: Influence of tacticity on phase transition from disordered to ordered state was investigated for isotactic polypropylenes (ipps) synthesized using Ziegler-Natta (ZN) and metallocene (M) catalysis method. The less stable and disordered rich ipps samples were annealed at higher temperature for few days. The more stable and ordered 2 fraction, average lamellae thickness (<l>) and melt temperature (T m ) as a function of annealing temperature (T a ) were studied by using solid state NMR, small angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC) techniques. The 2 fraction, crystals lamellae thickness and melt temperatures were strongly influenced by annealing temperature and tacticity of the ipps. The less stable and thinner 1 crystal having relatively lower melting temperature shows a significant increase of stability and thickening with annealing. The ordered fraction starts to increase with temperatures while the disordered phase partially melted. Thus, the melting of the disordered phase and simultaneous reorganization and/or re-crystallization into more ordered phase is considered for structural ordering and thickening of ipps under annealing conditions. Keywords: Isotactic Polypropylene, Phase Transition, Solid State NMR, Annealing Effect. 1. INTRODUCTION Isotactic polypropylene (ipp) is one of the most fundamental stereo-regular polymers that have been extensively investigated for academic and industrial interest [1-5]. They can be found indifferent polymorphs of and mesophase depending on their crystallization conditions. Among polymorphs, phase (monoclinic lattice) is generally form from both solution and bulk crystallization [2,6,7]. X-ray results showed that phase has two limiting packing structures such as 1 and phase. The less stable phase represent the full limit disorder structure with space group of C2/c and the more stable phase represent the full limit order structure with space group of P2 1 /c [4,8,9]. Although both phase have a common arrangement of helical structure but their methyl group arrangements are different. In case of phase, the orientations of methyl group are statistically ordered, and it is random for phase. The Borderless Science Publishing 46

2 nadian Chemical Transactions helical structure of both and form are well known [9-11]. It was reported that the commercial ipps has various degrees of disorder in the orientation of the methyl groups [4,9] indicating the presence of both 1 and 2 phases. However, it is possible to transform a structurally disordered phase to ordered phase by annealing or by crystallization at high temperatures between glass transition temperature (T g ) and melting temperature (T m ). This transformation process work by promoting the mobility of molecular chains toward a more regularized and thermodynamically more stable state (ordered state) of the cross linking of physical network of polymer. This increasing regularity or tacticity of the polymer, in turn, results in a profound enhancement of the mechanical properties [12,13] such as tensile strength, yield constant, and stiffness, which perhaps are the important parameters for industrial applications of the polymer. Therefore, it is very important to know the mechanism of this transition from the disordered phase to ordered phase quantitatively during annealing process with various tacticity for producing the appropriate polymer for different industrial applications. In previous study, the molecular ordering and dynamics of a commercial ZN ipp with 97% tacticity was reported under isothermal crystallization conditions [14]. From those observations, it was clear that a significant amount of disordered fraction remain in the system even after the sample was crystallized at very high temperature. Also, the influence of tacticity on local packing order in form of ipp under different annealing conditions was not studied even though it is highly probable that the relative stability of the 1 and phases in the presence of defects may influence during the annealing process. Therefore, in this work, we are reporting the influence of tacticity with stereo- and regio-defects in ipps on the ordering of the disordered phases under different annealing conditions. Since the fraction of ordering to up- and down-ward orientations of polymer chains is molecular events, the short range tools will be better to evaluate the phase transition. Solid state NMR is one of the very powerful tools for the characterization of local structure of polymers. Applying high power two-phase pulse modulated (TPPM) decoupling method, the 13 C NMR spectral resolution of the crystalline signals can be improved as described earlier [14]. This NMR approach allows quantitative analysis of the ordered fraction in ipps. SAXS will provide us information on crystal lamellae thickness due to annealing, while DSC heating run show the melting temperature to observe the significant change in melting of lamellae crystals. 2. EXPERIMENTAL Isotactic polypropylenes (ipps) with three different tacticity were synthesized using Ziegler-Natta catalyst and another ipp with both stereo- and regio-defect was synthesized using metallocene catalyst in the laboratory. Physical data of these polymers are provided in Table 1. Small amount of polymer was melted between two cover glasses on a hot stage for 5 min at 220 o C, a much higher temperature than equilibrium melt temperature to erase any previous thermal history. The thickness of the sample was kept constant to 0.2 mm by inserting metal spacer between the cover glasses. The molten polymer was rapidly quenched into icy water and then transfers to hot stage at 100 o C for one hour. This sample represents full limit disorder [9] and henceforth will be stated as standard 1 rich sample. This standard 1 rich sample is then allowed for annealing at higher temperature of 130, 140, 145, 150, 155, 160 and 165 o C for 24, 24, 48, 48, 48, 72 and 96 hours, respectively. All samples were prepared under nitrogen atmosphere to prevent thermal degradation. The Zeigler-Natta and metallocene samples are labeled as ZN and M, respectively (for example, the Ziegler-Natta catalyst sample-a annealed at 100 o C is labeled as ZNA_100 (Table 1)). Borderless Science Publishing 47

3 nadian Chemical Transactions Table 1. Weight average molecular weight M w, number average molecular weight M n, polydispersity index PDI as M w /M n and tacticity [mmmm] of ipps used in this study Sample s name [mmmm] M w 10 3 M n 10 3 PDI ZNA ZNB ZNC M Solid State NMR (SS-NMR) The 13 C SS-NMR experiments were carried out on a BRUKER AVANCE300 spectrometer, equipped with a 4 mm variable temperature (VT) cross-polarization (CP) magic angle spinning (MAS) NMR probe. The 1 H and 13 C carrier frequencies were and 75.6 MHz, respectively. The MAS frequency was set to 4000 ± 3 Hz. The 90 pulses for 1 H and 13 C were s. The recycle delay and CP time were 2 s and 1 ms, respectively. High power 1 H TPPM decoupling with field strength of 110 khz was used during an acquisition time of 160 ms. The chemical shift was referenced to the CH signal of adamantane (29.5 ppm) as an external reference. 13 C spin-lattice relaxation time in the laboratory frame (T 1C ) and T 1ρH measurements were used for selective observation of the pure crystalline signals of ipp [15]. In this study, a T 1ρH filter under a spin-locking (SL) field strength of 55 khz was used for a selective observation of the pure crystalline signals. All experiments were carried out at ambient temperature and pressure. 2.2 Small Angel X-ray Scattering (SAXS) SAXS studies were carried out using a Cu K radiation generated by a Rigaku Ultrax 4153A 172B X-ray diffractometer and a point-focusing SAXS camera. The camera length was 740 mm and the images were recorded using image plate (IP) with an exposure time of 2.5 h. Digitized data was then read from the IP using image plate reader. Using IP, a very small change in SAXS patterns could be obtained with a very short exposure time. The corrected pattern of an empty sample holder was subtracted from each pattern. To calculate the long period and lamellar thickness, a correlation-function method by Rigaku R-axis software was used. The crystalline forms of ipps were also examined by wide angle X-ray experiment and only form peak was detected. No additional peak was observed for different crystalline forms in any experiment at different annealing temperature, time and conditions. 2.3 Differential Scanning lorimetry (DSC) All the DSC heat flow data were collected with an updated computer interfaced Perkin-Elmer DSC-7 instrument. The DSC was calibrated for static temperature and thermal lag effects with indium and connected to an intra cooler to maximize heat transfer and to allow sub-ambient temperature control. Measurements of the melting points were performed at a heating rate of 10 o C/min with a typical sample weight of 5 ± 0.1 mg under nitrogen to prevent thermal degradation of the sample. 3. RESULTS AND DISCUSSION 3.1 Tacticity dependent melting behavior Two different types of samples (e.g., ZNA and M) with the same overall concentration of defects and masses were studied. It is well known that ZN ipp contain only stereo-defects and M ipp Borderless Science Publishing 48

4 Endotherm nadian Chemical Transactions contain both stereo- and regio-defects. ZN ipps have a broad molar mass distribution and non-uniform concentration of defects from chain to chain. On the other hand, M ipp have chains of narrow molar mass distribution and uniform distribution of defect concentrations along the chains [16]. Therefore, the differences in ordered fraction between ZNA and M sample are related to the distributions of defects in the structures. Figure 1 shows the melting endotherm of 1 rich sample, and annealed at 160 o C of ZN and M samples with a heating rate of 10 o C/min. Annealing of these 1 rich samples at 160 o C shows much broader and higher melt temperatures depending on their tacticity. This difference in melt temperature is related to two different phases with different size of the crystals: one is less stable with low melting temperature, and other is more stable with higher melting temperature. ZNA ZNB ZNC M Temperature ( o C) Figure 1. DSC melting endotherm for 1 form (thick solid curve) and annealed at 160 o C (thin curve) for ipps indicated in figure The final melt temperatures (end of DSC melting tail, T mf ) of the polymer also depend on their relative tacticity and annealing temperature (Figure 2). The figure shows that the final melting temperature of ZNA sample is higher (6 o C ) compare to that of ZNC sample which is related to the crystal defect present in ZNC where the polymer is loosely packed due to low tacticity. Similarly, sample with M catalyst shows lower final melt temperature due to the presence of induced stereo- and regiodefects in the structure even though it has the similar tacticity as of ZNA sample. After annealing at 160 o C, the final melt temperature shift to higher values showing a higher variation for low tactic sample rather than for high tactic sample. For example, ZNA shows a melt temperature shift of 7 o C whereas ZNC shows 18 o C for 1 sample and 1 sample annealed at 160 o C, respectively. On the other hand, M sample shows 17 o C higher melt temperature for annealed one compare to non-annealed one. From these results, it can be concluded that ipps with higher defect has large effect and lower defect shows less effect on annealing. Borderless Science Publishing 49

5 Endotherm T mf ( o C) nadian Chemical Transactions Tacticity Figure 2. Tacticity dependent final melt temperature for 1 form (open symbol) and annealed at 160 o C (solid symbol) for ZN (circle) and M sample (triangle) Heating data was collected for ZNA 1 rich sample annealed from 100 to 165 o C by using DSC to show a typical example of the variation in endotherm as a function of annealing temperatures (Figure 3). All melting endotherms show a broad peak consisting of at least two crystal populations, one for the melt of small crystals that related to standard 1 phase and another for re-crystallization of the sample during heating process. The melting curve shows an additional broader peak when annealed above 130 o C and also a shift to the higher temperature. This is related to the initial melting of 1 rich sample started around 140 o C. Annealing above 140 o C for long time generate more stable 2 crystal due to the partial melting of less stable 1 crystals at that temperature. 165 o C 160 o C 155 o C 150 o C 140 o C 130 o C 100 o C Temperature ( o C) Figure 3: DSC melting endotherm for ZNA sample as a function of annealing temperature Borderless Science Publishing 50

6 <T m > ( o C) nadian Chemical Transactions ZNA ZNB ZNC M T a ( o C) Figure 4. Annealing temperature dependent average melt temperature for different ipps Figure 4 shows the annealing temperature dependent average melt temperature with a heating rate of 10 o C/min for ZN and M ipps. The average melt temperature was determined by deconvulation of the melting peaks (area of the deconvulated peaks were multiplied by the peak temperature). The average melt temperature for 1 rich samples always shows minimum and increases with annealing temperature. At lower annealing temperature, the average melting point does not increase significantly until 140 o C, and then increases with annealing temperature. The average melting temperature for M sample is always lower than that of ZN samples. 3.2 Temperature dependent ordered fraction Using the method described in reference 4, the ordered fraction was estimated based on the CPMAS spectrum by SS-NMR. The spectral resolution was enhanced by optimizing the parameter in NMR and the 2 fraction was calculated using the method. Details of optimizing the parameter in NMR have previously been reported [14]. CH 165 o C 160 o C 155 o C 150 o C 145 o C 140 o C 130 o C 100 o C 1 rich CH 3 CH Chemical Shift /ppm Figure C CPMAS NMR spectra for ZNA sample as a function of annealing temperatures. The lowest spectrum is for the standard 1 form and the highest spectra are for the annealed sample at 165 o C Borderless Science Publishing 51

7 2 (%) nadian Chemical Transactions Figure 5 shows typical spectra of 13 C CPMAS NMR for the crystalline region of 1 rich sample of ZNA as a function of annealing temperatures. The 1 rich sample shows very broad (~22, ~26 and ~44 ppm) signal for all functional signals for CH 2, CH and CH 3, corresponding to full limit disorder structure. The observed broad signals are attributed to heterogeneous line broadening due to variations of local upward and downward stem orientations of methyl group arrangements. Small doublet appears for all functional groups of CH 2, CH and CH 3 corresponding to α 2 [14,17] and superimpose over the broad α 1 signals when annealed at T a = 130 C. The intensities of all functional group related to the α 2 signals increase and become narrower with increasing annealing temperatures. The observed doublets are related to the ordering of the packing structure [18,19]. This observation is well consistent with previously reported NMR results [5,17,20]. The contrast between α 2 and α 1 shapes allows an easy evaluation of α 2 fraction which at a given T a is determined by integrating the peak area of CH 2 signals in the spectral region where signals of α 1 and α 2 forms are well separated ZNA ZNB ZNC M T a ( o C) Figure 6. Annealing temperature dependent ordered fraction as a function of defects in ipps. To study the annealing effect on ordered fraction, α 2 fraction of ZNA, ZNB, ZNC and M samples was calculated as a function of annealing temperature (Figure 6). For all annealed samples the ordered fraction, α 2, started with an initial value, remains almost same until 130 o nd then started to increase with higher annealing temperature. For example, α 2 fraction of ZNA sample with an initial value of 6-7% shows no significant changes until 130 o C, then steadily increases with higher annealing temperatures and raised to 51% at 165 o C. This result is consistent with previous DSC melting experiment where at lower annealing temperatures a little fraction of α 1 rich crystals melted (showing lower mobility) and accounts for lower amount of transformation to α 2 fraction. At higher annealing temperature, melting of α 1 crystals increases and also the transformation to thermodynamically stable α 2 form. The maximum amount of formation of α 2 fraction strongly depends on the tacticity of the polymer. A maximum of 51, 49, 42 and 38% α 2 fraction is estimated in case of ZNA, ZNB, ZNC and M sample annealed at 165, 160, 160 and 155 C, respectively. The absolute value of ordered fraction depends on the crystallization process, tacticity, temperatures, and may even differ with the technique used for estimation. For example, a much higher ordered fraction (~100% [21] and 90-95% [10]) were obtained for isothermal Borderless Science Publishing 52

8 <l> (nm) nadian Chemical Transactions crystallization of ipp measured by XRD analysis. The higher reported values might be related to isothermal crystallization process where the crystallization process approaches from molten state. Our experimental result shows lower ordered fraction due to annealing process where the ordering process approaches from pre-existing disordered phase. 3.3 Temperature dependent lamellae thickness Average lamellae thickness was calculated by examining the SAXS patterns taken at room temperature using correlation function described earlier [15]. All 1 rich samples and annealed at different temperatures the average lamellae thicknesses are plotted against annealing temperatures (Figure 7). For samples annealed at low temperature, lamellae is thinner (~13 nm), and average lamellae thickness remains almost same until 130 o C, and become thicker (~57 nm for highest annealed temperature) with increasing temperature. The thickening of the lamellae is related to the melting temperature of 1 rich sample during annealing process. Higher annealing temperature provides more melting fraction of 1 rich sample, higher mobility of polymer chain which yields to the thickening of the lamellae. Therefore, at a constant annealing temperature the crystal lamellae are thicker for lower tacticity sample and vice versa. The average lamellae thickness <l>, crystalline thickness l c, and amorphous thickness l a with annealing temperatures for different ipps are listed in Table 2. The SAXS data tabulated in table indicate that the average lamellae thickness <l>, crystalline thickness l c, and amorphous thickness l a increases with annealing temperatures. Table 2. Lamellae characteristics of the polymers used in this study with an error range of ±3% T a ZNA ZNB ZNC M <l> l c l a <l> l c l a <l> l c l a <l> l c l a ZNA ZNB ZNC M T a ( o C) Figure 7. Annealing temperature dependent average lamellae thickness of different ipps Borderless Science Publishing 53

9 <l> (nm) l c (nm) nadian Chemical Transactions t a (h) Figure 8. Time dependent crystalline lamellae thickness of ZNC_155 sample annealed at 155 o C from 1 rich sample ZNA ZNB ZNC M (%) Figure 9. Average lamellae thickness variation with 2 fraction as a function of tacticity of different ipps The annealing time is also an important factor to melt fraction of the 1 rich sample that gives yields to transformation to 2 fraction. Figure 8 shows annealing time dependent crystalline lamellae thickness, l c, for a typical ZNC sample annealed at 155 o C. It is evident that crystalline lamellae become thicker within 10 hrs and remain same for more annealing time, a much shorter time taken than that of the annealing time described in the experimental section. 3.4 Crystal thickness and ordered fraction Figure 9 shows 2 fraction and average lamellae thickness, <l>, as a function of tacticity. Initial lamellae thickness increases only a few nm in range with a low value of ordered fraction of the polymer but in higher ordered region,<l> undergoes a significant increase, which, also indicate the strong Borderless Science Publishing 54

10 nadian Chemical Transactions influence of local ordering and thickening of the lamellae under annealing. The observed relationship between <l> and 2 fraction suggests that a further increase of annealing temperatures should results in much thicker lamellae in ordered packing areas of the polymer. This view is also supported by earlier studies which showed that ordered packing areas have thicker lamellae than that for disordered packing areas [14]. 4. CONCLUSION The influence of tacticity on structural ordering of ipps was investigated at different annealing conditions. Thinner lamellae can be obtained from 1 rich sample or from sample annealed at lower temperature and relatively thicker lamellae at higher annealing temperatures. At higher annealing temperatures the polymer chain gives higher 2 fraction (ordered state) that leads to significant development of lamellar thickness and melt temperatures. The formation of 2 fractions are also related to tacticity and melting temperature of the less stable crystal. The enhancement of ordering with annealing will therefore highly impact the mechanical properties of the polymer as tensile strength, yield constant, and stiffness are a function of ordered fraction. The influence on mechanical properties by annealing process will be one of the topics of future issue and will be published elsewhere. SUPPORTING INFORMATION Details of ipp structure and synthesis process are included in the supporting information. This document is available at ACKNOWLEDGMENT This work was partially done in Japan and financially supported by Japan Industrial Technological Association (JITA) Tsukuba, Japan. The authors are grateful to Dr. M. Muthukumar, UMASS, for his helpful discussions. REFERENCES AND NOTES [1] Cong, Y.; Hong, Z.; Zhou, W.; Chen, W.; Su, F.; Li, H.; Li, X.; Yang, K.; Yu, X.; Qi, Z.; Li, L.: Conformational Ordering on the Growth Front of Isotactic Polypropylene Spherulite. Macromol. 2012, 45, [2] De Rosa, C.; Auriemma, F.; Di Girolamo, R.; Ruiz de Ballesteros, O.; Pepe, M.; Tarallo, O.; Malafronte, A.: Morphology and Mechanical Properties of the Mesomorphic Form of Isotactic Polypropylene in Stereodefective Polypropylene. Macromol. 2013, 46, [3] De Rosa, C.; Auriemma, F.; Ruiz de Ballesteros, O.; De Luca, D.; Resconi, L.: The Double Role of Comonomers on the Crystallization Behavior of Isotactic Polypropylene: Propylene-Hexene Copolymers. Macromol. 2008, 41, [4] Mencik, Z.: Crystal structure of isotactic polypropylene. J. Macromol.Sci. Part B 1972, 6, [5] Nakamura, K.; Shimizu, S.; Umemoto, S.; Thierry, A.; Lotz, B.; Okui, N.: Temperature Dependence of Crystal Growth Rate for [alpha] and [beta] Forms of Isotactic Polypropylene. Polym. J. 2008, 40, [6] Karger-Kocsis, J.; Martuscelli, E.: Structure and properties of polypropylene-elastomer blends. In Polypropylene Structure, blends and composites; Springer Netherlands, 1995; pp Borderless Science Publishing 55

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