Comparative study of the kinetics of non-isothermal melt solidification of random copolymers of butene-1 with either ethylene or propylene

The kinetics of non-isothermal melt solidification of random butene-1/propylene copolymers has been compared with that of random butene-1/ethylene copolymers. Analysis of the distance between neighbored chain segments in the crystal phase revealed inclusion of propylene chain defects into crystals, while ethylene co-units are excluded from crystallization. As a consequence of different acceptance of propylene and ethylene chain defects to participate in crystallization, the kinetics of the transition of the melt into ordered phase is significantly slower in random butene-1/ethylene copolymers. For samples of similar co-unit concentration, the decrease of the crystallization temperature and of the critical cooling rate to suppress ordering/crystallization is higher in random butene-1/ethylene copolymers than in butene-1/propylene copolymers. Due to the required rejection of ethylene co-units at the crystal growth front, ultimately, the maximum crystallinity is lower in butene-1/ethylene copolymers than in butene-1/propylene copolymers of similar amount of co-units.     

Effect of Linear and Ring-like Co-units on the Temperature Dependence of Nucleation and Growth in II-I Phase Transition of Butene-1 Copolymers

The phase transition from tetragonal form II to hexagonal form I was studied for the butene-1/ethylene and butene-1/1,5-hexadiene random copolymers, which have comparable molecular weight but distinct linear ethylene and ringlike methylene-1,3-cyclopentane (MCP) structural co-units, respectively. It is known that this solid phase transition follows the nucleation-growth mechanism, so the stepwise annealing protocol was utilized to investigate the influences of co-units on the optimal nucleation and growth temperatures. Compared with optimal nucleation and growth temperatures of ?10 and 35 °C, respectively, in polybutene-1 homopolymer, two butene-1/ethylene copolymers with 1.5 mol% and 4.3 mol% co-units have the slightly lower optimal nucleation temperature of ?15 °C but much higher optimal growth temperature of 50 °C. Clearly, the effect of ethylene co-unit is more significant on varying optimal temperature for growth than for nucleation. Furthermore, when the incorporated co-unit is ringlike MCP, the optimal nucleation temperature is ?15 °C for 2.15 mol% co-units, the same with above BE copolymers, but ?13 °C for a very low concentration of 0.65 mol%. Interestingly, the optimal growth temperature of butene-1/1,5-hexadiene copolymers with 0.65 mol%?2.15 mol% MCP counits increases to 55 °C, which is also independent on co-unit concentration. These obtained values of optimal temperatures provide crucial parameters for rapid II-I phase transition.     

New gelators of urea-containing triazine derivatives: effects of aggregation and optical features in different organic solvents

New gelators for urea-containing triazine derivatives were synthesised, and their gelation potential was examined using different organic solvents. These compounds were found to form the organogels with a variety of organic solvents, such as hexane and other solvents. The elongated alkyl tails of the gelators displayed an obvious decrease in the critical gelation concentrations of apolar solvents and an increase in the compatibility of gelation in polar solvents. The resulting thermo-reversible gels were characterised by using the dropping ball method and a number of other instruments. The melting temperature (Tm) of the gels in decalin and CCl₄ increased with the gelator concentrations. The intermolecular hydrogen bonding of gelation in different organic solvents was observed using an FT-IR spectrometer. Temperature-dependent UV–vis and fluorescence analysis showed that the organogels displayed diverse aggregations and various fluorescence effects in different organic solvents. Blue fluorescence and J-aggregation in decalin and the quenched effect and π–π stacking in CCl₄ were observed. Further, the morphological self-assembled feature in different organic solvents was studied with a scanning electron microscope, and the morphological features demonstrated that there were different aggregations in different solvents. In conductivity electrolyte experiments, the organogel electrolytes exhibited high conductivity (σ) compared with the corresponding tetrabutylammonium perchlorate (TBAP)/THF solution. The conductivity of the gel electrolytes increased with the concentration of the electrolyte salts and temperature. When the sol–gel temperature was achieved, a high ion conductivity was observed compared with the corresponding TBAP/THF solution. When the ratio of the added electrolyte salts exceeded 5%, gelation was inhibited. Furthermore, the effect of the electrolyte salts on the Tm of the gel was confirmed. The added electrolyte salts affected the gelation ability, but did not affect the sol–gel temperature.     

Synthesis and gelation properties of PEG-PLA-PEG triblock copolymers obtained by coupling monohydroxylated PEG-PLA with adipoyl chloride

Ring-opening polymerization of D,L-lactide was carried out in the presence of monohydroxylated poly(ethylene glycol) (PEG) with Mn of 2000 and 5000, using zinc powder as catalyst. The resulting PEG-b-polylactide (PEG-PLA) diblocks with various ethylene oxide/lactyl (EO/LA) ratios were coupled with adipoyl chloride to yield PEG-PLA-PEG triblock copolymers. N-Dimethylaminopyridine (DMAP) was used as catalyst. The obtained PEG-PLA-PEG triblock copolymers were characterized by various analytical techniques such as IR, 1H NMR, size exclusion chromatography, X-ray diffraction, and differential scanning calorimetry. Data showed that all the copolymers were semicrystalline with the PEG-type crystalline structure, the crystallinity decreasing with increasing PLA block length. Bioresorbable hydrogels were prepared from the water-soluble triblock copolymers. Rheological measurements showed a gel-sol transition with increasing temperature and gelation was found to be thermoreversible. The copolymer solution behaves like a viscoelastic liquid above the gel point and like a viscoelastic solid below the gel point. The critical gelation concentration, the gel-sol transition temperature at a given concentration, and corresponding moduli depend on both the EO/LA ratio and the molecular weight of the copolymers. It is assumed that gelation results from interactions between PEG blocks at low temperatures and that these interactions are disrupted as the temperature is elevated. The shrinking of PEG blocks with increasing temperature seems to be in agreement with the variation of the gel-sol transition temperatures.     

A water-soluble PPDO/PEG alternating multiblock copolymer: Synthesis, characterization, and its gel-sol transition behavior

Biodegradable and nontoxic alternating multiblock copolymers based on poly (p-dioxanone) (PPDO) and poly (ethylene glycol) (PEG) were synthesized by the coupling reaction of two bifunctional prepolymers, a dihydroxyl-terminated PPDO and dicarboxylated PEG. The prepolymers and the resulting PPDO/PEG multiblock copolymers were characterized by various analytical techniques such as FT-IR, ¹H NMR, GPC, DSC and TG. At high concentration levels above critical gelation concentration (CGC), the aqueous solution of copolymers formed a gel. Temperature-sensitive gel to sol transition behaviors were investigated by the test tube inverting method. Dynamic light scattering (DLS) was used to investigate the micelle of copolymers, whose association probably caused the gelation of the system. Therefore, this novel copolymer has a great potential in injectable drug-delivery system for long-term delivery of drugs.     

Gelation and precipitation of copolymers from solution

There are several reports in the literature to the effect that when isotactic polystyrene crystallizes from moderately dilute solution, lamellas (precipitate) form at high crystallization temperatures and gels at low temperatures. We report here the occurrence of a very similar phenomenon during the crystallization of ethylene-butene-1 copolymers. The observations for the latter system can be given a straightforward explanation based on the demonstrated compositional fractionation that occurs on crystallization. Low co-unit content copolymers are associated with lamellas. Gels invariably contain much higher co-unit content. A generalization of these findings is invoked to explain the previous polystyrene results. A new resonance detected by ¹³C NMR experiments in this polymer can be ascribed to head–head sequences.     

Micelles and gels of mixed triblock copoly(oxyalkylene)s in aqueous solution

The micellization in dilute aqueous solution of a 50/50 wt% mixture of two triblock copolymers, E45B14E45 and E62P39E62, and the gelation of concentrated micellar solutions have been investigated over a range of temperatures. Here E, B, and P denote oxyethylene, oxubutylene, and oxypropylene chain units. Comparison is made with aqueous solutions of the individual copolymers. The results of light scattering measurements are consistent with effectively separate micellization of the two copolymers in the mixture. Hard gel formed when the extent of micellization was high for both copolymers. Because of the relatively high critical micellization temperatures of copolymer E62P39E62, the low-temperature boundary of the hard gel was high for this copolymer and for the mixture. The minimum concentration for hard-gel formation was higher for the mixture than for either of the individual copolymers, as would be expected for packing of two distributions of micelles of different average size.     

Gelation of a Highly Fluorescent Urea‐Containing Triarylmelamine Derivative: 2,4,6‐Tris(p‐N′‐Octadecylureido‐Anilino)Triazine in Organic Solvents

A new gelator of urea‐containing triazine derivatives was synthesized and tested in order to explore the gelation potential in different organic solvents. This compound has been found to form organogels with a variety of organic solvents such as decalin and other solvents. The resulting thermo‐reversible gel was characterized by using the dropping ball method and a number of other instruments. The melting temperature of the gel increased with the gel concentration. The intermolecular hydrogen bonding of gelation was demonstrated through an FT‐IR spectrometer. UV‐Vis and fluorescence analysis showed that the gel displayed various optical effects in different organic solvents. The blue fluorescence of the gel in decalin and the quenched effect of gel in CHCl³ were displayed, respectively. Morphological features in decalin and CHCl₃ were studied by applying atomic force microscopy (AFM), and the morphological features demonstrated that there were different aggregations in different solvents. In conductivity electrolyte experiments, the organogel electrolytes indicated high conductivity (σ) comparable to the corresponding NaClO₄/THF solution. The conductivity of gel electrolytes was increased with electrolyte salt.     

Micellization and Thermogelation of Poly(ether urethane)s Comprising Poly(ethylene glycol) and Poly(propylene glycol)

A series of multiblock poly(ether urethane)s comprising poly(ethylene glycol) (PEG), and poly(propylene glycol) (PPG) segments were synthesized. Their aqueous solutions exhibited thermogelling behavior at critical gelation concentrations (CGC) ranging from 8 to 12 wt%. The composition and structural information of the copolymers were studied by GPC and ¹H NMR. The critical micellization concentration (CMC) and thermodynamic parameters for micelle formation were determined at different temperatures. The temperature response of the copolymer solutions were studied and found to be associated with the composition of the copolymers.     

Tensile yield of polyethylene in relation to crystal thickness

Tensile yield stress measurements have been performed on a serie of four ethylene/1-butene copolymers covering the crystallinity range 0.33–0.74. Samples have been prepared from melt-crystallization and from solution-crystallization at various concentrations in decalin in order to span a wide range of crystal thickness. It is shown that tensile yield stress depends on crystal thickness rather than on crystallinity. This result is perfectly consistent with a plasticity model based on the nucleation of dislocations. Besides, it in-dicates that the main role of the amorphous phase is only to transmit the load upon the crystals, irrespective of its proportion. The yield stress dependence on crystallinity reported previously relies on the direct correlation of crystal thickness and crystallinity, in the case of melt-crystallized samples only. Discrepancies with the model are amply discussed. © 1993 John Wiley & Sons, Inc.     

Synthesis and Self-Association of Stimuli-Responsive Diblock Copolymers by Living Cationic Polymerization

The living cationic polymerization of several functional monomers in the presence of an added base is investigated as a possible preparation of a new series of water-soluble or stimuli-responsive copolymers. Under appropriate conditions, the polymerization allows the selective preparation of polymers with various shapes and different sequence distributions of monomer units, including stimuli-responsive block copolymers, gradient copolymers, poly(vinyl alcohol) graft copolymers, and star-shaped polymers. The stimuli-induced self-association of the diblock copolymers is also examined. An aqueous solution of the diblock copolymer with a thermo-sensitive segment undergoes rapid physical gelation upon warming to the critical temperature to give a transparent gel, and returns sensitively to the solution state upon cooling. The sharp transition of stimuli-responsive segments with highly controlled primary structure turns out to play an important role in the self-association. Small-angle neutron scattering, dynamic light scattering, and electron microscopy studies reveal that the physical gelation involves a thermosensitive micellization of diblock copolymers (core size: 18-20 nm) and subsequent micelle macrolattice formation (bcc symmetry). Based on the gelation mechanism, several stimuli-responsive gelation systems are achieved using other stimuli such as the addition of a selective solvent or compound, cooling, pH change, and irradiation with ultraviolet light.     

Mechanisms of micellization and rheology of PEO-PPO-PEO triblock copolymers with various architectures

Micelle formation was followed by micro-DSC and rheology for aqueous solutions of two copolymers of PEO-PPO-PEO, the Pluronic F127 (from BASF) and the EG56 (from PolymerExpert), a branched copolymer built with three chains of F127 type. It is shown that micellization is endothermic and that, for both polymers, the enthalpy of formation/melting is proportional to total concentration. The rheology of the solutions was carefully analyzed, before gelation for F127, and it reveals firstly the progressive changes of solubility of the unimers (decease of relative solution viscosity), followed by micelle formation over a 10 degrees C range. In this range, the micelle concentration dependence on temperature was deduced from enthalpy measurements and the corresponding volume fractions were derived. Viscosity was interpreted within the framework of well-known theories for hard sphere suspensions (Krieger-Dougherty or Quemada) based on an analogy between micelles and nanosized hairy grain suspensions. The gel state is achieved due to formation of the colloidal crystal. For EG56, the rheology is quite different; as the aggregation increases with temperature, a progression is observed from Newtonian to visco-elastic liquid. The characteristic frequency, defined by the relation G(') = G('), for EG56 varies with temperature and the corresponding times increase by two orders of magnitude according to an Arrhenius law. The frequency dependence of G(') and G(') at different temperatures can be superposed with a horizontal shift factor and a small amplitude adjustment. There is no elastic solid formation in this case. The "gelation" of these two copolymers is compared to the physical gelation of cold-set gels (gelatin).     

In situ-gelling, erodible N-isopropylacrylamide copolymers

Copolymers of N-isopropylacrylamide, 2-hydroxyethyl methacryl lactate [(HEMA)-lactate] and acrylic acid (AAc) were prepared with varying mole ratios of monomers to develop copolymers with gelation properties above a certain concentration for a bioerodible, in-situ gelling material. The copolymers formed gels in situ under physiological condition. The gelation temperature of the copolymers decreased as the HEMA-lactate content of the copolymers increased due to the hydrophobicity of HEMA-lactate, and increased as the AAc content increased due to the hydrophilicity of AAc. The gels redissolve at 37 degrees C as their LCSTs increase above 37 degrees C due to the hydrolysis of the HEMA-lactate pendant groups.     

Thermogelling of double hydrophilic multiblock and triblock copolymers of N,N-dimethylacrylamide and N-isopropylacrylamide: chain architectural and Hofmeister effects

A series of thermoresponsive double hydrophilic (AB)(n) multiblock and ABA triblock copolymers of N,N-dimethylacrylamide (DMA) and N-isopropylacrylamide (NIPAM) with varying sequence lengths were synthesized via successive reversible addition-fragmentation chain transfer (RAFT) polymerizations by employing polytrithiocarbonate as the chain transfer agent. Previously, we reported that multiblock copolymers in dilute aqueous solutions can form either unimolecular or multimolecular micelles at elevated temperatures depending on the relative chain lengths of PDMA and PNIPAM sequences (Zhou et al. Langmuir 2007, 23, 13076-13084). In this follow-up work, we further explored and compared the chain architectural (multiblock vs triblock) and Hofmeister effects (addition of various sodium salts) on the gelation behavior of multiblock and ABA triblock copolymers at high concentrations and attempted to establish a correlation between the aggregation behavior and gelation properties of multiblock copolymers at low and high polymer concentrations, respectively. It was found that only m-PDMA(p)-PNIPAM(q) multiblock copolymers with PDMA and PNIPAM sequence lengths located within a specific range can form physical gels at elevated temperatures. Rheology measurements revealed that multiblock copolymers possess considerably lower critical gelation temperatures (CGT) and higher gel storage modulus, G'(gel), as compared to those of PNIPAM-b-PDMA-b-PNIPAM triblock copolymers possessing comparable sequence lengths. The addition of inorganic sodium salts can effectively facilitate thermogelling for multiblock and triblock copolymers, resulting in decreasing CGTs and critical gelation concentrations (CGCs) in the order of Hofmeister series with increasing hydration capabilities. The unique thermogelling behavior of aqueous multiblock copolymer solutions in the absence and presence of inorganic salts, as compared to that of ABA triblock copolymers, augurs well for their potential applications in various fields such as biomaterials and biomedicines.     

A quantitative electron microscopic study of the crystalline structure of ethylene copolymers

Quantitative thin-section electron microscope studies were made on compositional fractions of ethylene copolymers crystallized under several extreme conditions. The co-units chosen were such that the chains contained either ethyl, hexyl, or acetate side groups. Certain general features are independent of the chemical nature of the co-unit. It was found that up to about 2 mol % co-unit content lamellae are observed in all copolymers which were slowly cooled from the melt. The lateral dimensions of lamellae are not as large as in corresponding homopolymers and the lamellae are segmented. As the branch content increases the lamellae become shorter and eventually degrade into small crystallites. Within this broad outline of behavior, differences between the copolymer types are discerned and discussed. By comparison with the Raman LAM it is shown that quantitative values for the crystallite size distribution are obtained by the electron microscopy method used. Furthermore, when the specimen consisted of well-ordered lamellar stacks the degree of crystallinity obtained from the electron micrographs was in quantitative agreement with the enthalpy of fusion.     

Novel random copoly(arylene ether-thioether ketone)s based on 2,2-bis(4-mercaptophenyl)propane and 4,4′-dihydroxybiphenyl: Synthesis and properties

A series of novel poly(arylene ether-thioether ketone) random copolymers were synthesised by copolymerisation of 4,4′-difluorobenzophenone with bis(N,N-dimethyl-S-carbamate) of 2,2-bis(4-mercaptophenyl)propane (MBTA) and 4,4′-dihydroxybiphenyl (BP). The syntheses were performed using a one pot polymerisation method starting from different mole percentages of BP with respect to MBTA, varying from 20% to 80%. The copolymers were obtained in good yields and had molecular weights in the range 10,000–17,000 as determined by GPC. The structural proof was provided by ¹H-NMR spectroscopy. In order to study the effect of the copolymerisation on the thermal properties and crystal behaviour, also the poly(ether biphenyl ether ketone) homopolymer (PEBEK) has been synthesised using the same polymerisation method. The TGA analysis shows that the high stability of PEBEK is only slightly affected by the introduction of co-units in the chain. The copolymers with an amount of PEBEK variable from 80 to 50 mol% are crystalline, with percentage of crystallinity and melting temperature which decrease as the co-unit content increases. The samples with lower amount of PEBEK (40 and 20 mol%) are amorphous. The T_g values tend to decrease with the reduction of the percentage of PEBEK units.     

Morphology and mechanical properties of ultrahigh molecular weight polypropylene prepared by gelation/crystallization at various temperatures

Films of ultrahigh molecular weight (5.4×10⁶) polypropylene were produced by gelation/crystallization at various temperatures from dilute decalin solutions according to the method of Smith and Lemstra. The temperatures chosen were 20°, 30°, 50°, and 60°C. With increasing the temperature, the long period and crystallinity of the resultant gel film increased. By contrast, when the films were stretched up to 50 } 60 times, the increases in Young's modulus and crystallinity become more significant, as the temperature of the gelation/crystallization became lower. This interesting phenomenon is thought to be due to the dependence of the number of entanglements on the temperatures concerning gelation/crystallization and evaporation of solvent from the gel to form a film.     

Synthesis and characterization of biocompatible, thermoresponsive ABC and ABA triblock copolymer gelators

The synthesis of doubly thermoresponsive PPO-PMPC-PNIPAM triblock copolymer gelators by atom transfer radical polymerization using a PPO-based macroinitiator is described. Provided that the PPO block is sufficiently long, dynamic light scattering and differential scanning calorimetry studies confirm the presence of two separate thermal transitions corresponding to micellization and gelation, as expected. However, these ABC-type triblock copolymers proved to be rather inefficient gelators: free-standing gels at 37 degrees C required a triblock copolymer concentration of around 20 wt%. This gelator performance should be compared with copolymer concentrations of 6-7 wt% required for the PNIPAM-PMPC-PNIPAM triblock copolymers reported previously. Clearly, the separation of micellar self-assembly from gel network formation does not lead to enhanced gelator efficiencies, at least for this particular system. Nevertheless, there are some features of interest in the present study. In particular, close inspection of the viscosity vs temperature plot obtained for a PPO43-PMPC160-PNIPAM81 triblock copolymer revealed a local minimum in viscosity. This is consistent with intramicelle collapse of the outer PNIPAM blocks prior to the development of the intermicelle hydrophobic interactions that are a prerequisite for macroscopic gelation.     

含有3H-苯并吡喃-2-酮骨架化合物的液晶性和凝胶性能研究

在化合物1-n凝胶性能研究结果的基础上,对其化学结构进行了修饰,在烷氧链一侧增加了1个苯环,从而合成了4-烷氧基-双苯基-4′-羧酸酯(4-alkoxy-biphenyl-4′-carboxy-lates,2-n）化合物。 研究了化合物2-n的液晶性和凝胶性能,与化合物1-n相比,化合物2-n显示互变的向列相,并且能够同多种有机溶剂形成凝胶。 测试了凝胶剂在各种溶剂中的临界凝胶浓度。 扫描电子显微镜观察结果表明,干凝胶呈纤维状三维网络结构。 在凝胶的形成中,由于3H-苯并吡喃-2-酮衍生物的内酯骨架具有较大的宽度和偶极矩,因而引起偶极作用。 偶极作用是3H-苯并吡喃-2-酮衍生物具有凝胶性能的原因。     

PEO-PPO-PEO水溶液体系胶束化及凝胶化行为的耗散粒子动力学模拟

采用耗散粒子动力学(Dissipative particle dynamics, DPD)模拟方法研究了三嵌段共聚物聚氧乙烯-聚氧丙烯-聚氧乙烯(PEO-PPO-PEO)的胶束化和凝胶化行为. 通过模拟得到了F127(EO₉₉PO₆₅EO₉₉)水溶液的临界胶束浓度和临界凝胶浓度. 结果发现, 在298 K、 质量分数低于40%时, F127水溶液中形成的胶束形状均为球形. 此外,进一步研究了亲水嵌段长度对胶束结构及凝胶形成浓度的影响, 结果发现, 亲水嵌段越短, 越有利于长椭球状胶束的形成, 而临界凝胶浓度随着亲水嵌段PEO长度的增加而降低.