Tailoring the Crystallization Behavior and Mechanical Property of Poly(glycolic acid) by Self-nucleation

Biocompostable poly(glycolic acid)(PGA)crystallizes slowly under fast cooling condition,leading to poor mechanical performance of the final products.In this work,a self-nucleation(SN)route was carried out to promote the crystallization of PGA by regulating only the thermal procedure without any extra nucleating agents.When self-nucleation temperature(Ts)decreased from 250℃ to 227℃,the nuclei density was increased,and the non-isothermal crystallization temperature(Tc)of PGA was increased from 156℃ to 197℃ and the half-life time(t0.5)of isothermal crystallization at 207℃ was decreased by 89%.Consequently,the tensile strength and the elongation at break of the PGA were increased by 12%and 189%,respectively.According to the change of Tc as a function of Ts,a three-stage temperature domain map(Domain I,II and III)was protracted and the viscoelastic behavior of the self-nucleation melt and the homogeneous melt was studied.The results indicated that interaction among PGA chains was remained in Domain IIb,which can act as pre-ordered structure to accelerate the overall crystallization rate.This work utilizes a simple and effective SN method to regulate the crystallization behavior and the mechanical properties of PGA,which may broaden the application range of resulting materials.

Microstructure Characterization of Short-chain Branching Polyethylene with Differential Scanning Calorimetry and Successive Self-nucleation/Annealing Thermal Fractionation

A series of the copolymers of ethylene with 1-hexene （M1-M9） synthesized by metallocene catalyst Et[Ind]2ZrC1JMAO was studied by differential scanning calorimetry and successive self-nucleation and annealing （SSA） thermal fractionation. The distribution of methylene sequence length （MSL） in the different copolymers was determined using the SSA method. The comonomer contents of samples M4 and M5 are 2.04 mol% and 2.78 mol%, respectively. Both M4 and M5 have low comonomer content and their MSL distribution profiles exhibit a monotonous increase trend with their MSL. The longest MSL of M5 is 167, and its corresponding molar percent is 43.95%, which is higher than that of M4. Moreover, the melting temperature （Tm） of M5 is also higher than that of M4. The comonomer contents of samples M7, M8, and M9 are 8.73 mol%, 14.18 mol% and 15.05 mol%, respectively. M7, M8, and M9 have high comonomer contents, and their MSL distribution profiles display unimodality. M7 has a lower peak value of 33 and a narrow MSL distribution, resulting in a Tm lower than that of M8 and M9. The MSL and its distribution are also key points that influence the melting behavior of copolymers. Sometimes, MSL and its distribution of copolymers have a greater impact on it than the total comonomer contents, which is different from traditional views.

Self-nucleation of Patterned Polymer Thin Films Defined by Soft Lithography

The nucleation of crystals is often a determining step in the phase transition of materials,but it remains a challenge to control the density and specific location of nuclei simultaneously.Here we fabricated the isolated single crystals of uniform size with controlled number density and spatial distribution by self-nucleation of patterned dendritic crystals.Imprint lithography creates the periodic void space on the surface of poly(ethylene oxide)-b-poly(2-vinyl pyridine)(PEO-b-P2VP)block copolymer thin films and provides spatial redistribution of polymers,leading to the preferential nucleation and subsequent oriented growth of dendrites in the periodic arrays of imprinted lines.The morphology and thermal stability of the patterned crystals can be adjusted by tuning embossing conditions(e.g.,temperature and pressure).Furthermore,in the self-nucleation technique,the annealing temperature and heating rate are used as the feedback parameters to map the number density and spatial distribution of regrown single crystals.Such PEO-b-P2VP crystalline pattern can be used as a versatile template for large-area manufacturing of selective metal patterns for electronic devices and other applications.

Application of Successive Self-nucleation and Annealing to Study the Stereo-defects Distribution of Biaxially Oriented Polypropylene

The stereo-defects distribution of polypropylene of the two industry biaxially oriented polypropylene （BOPP） samples T28FE and F28SO with different processing properties was studied through successive self-nucleation and annealing （SSA） technique. It was found that there were more medium isotactic components in sample F28SO, and the isotactic sequence length of polypropylene of sample F28SO was shorter and the isotactic sequence length distribution of polypropylene of sample F28SO was broader, which could be processed well at high-speed orientation during the processing of BOPP films. This result indicates that the isotactic sequence length distribution of polypropylene is related to the processing speed during preparing BOPP films, and the stereo-defects distribution of polypropylene has an important influence on its processing ability.

NUCLEATION AND CRYSTALLIZATION OF H-SHAPED(PS)_2PEG(PS)_2 BLOCK COPOLYMERS

A series of H-shaped （PS）2PEG（PS）2 block copolymers with different PS chain lengths were prepared. The influence of different confinements active on the crystallization and self-nucleation （SN） behavior of the PEG blocks was investigated by differential scanning calorimetry （DSC）. When the content of the crystalline block was high, a classical SN behavior was obtained. The block copolymer with PEG content of 49% （by weight） showed a classical SN behavior with a narrow self-nucleation domain and had bimodal crystallization exotherms. When the PEG dispersed as separated microdomains in the block copolymer, the self-nucleation domain disappeared and only annealing was observed.

Interfacial Chain Entanglements Induced Melt Memory Effect in Poly(ε-caprolactone)/Silica Nanocomposites

It is known that the enhanced melt memory effect is strongly correlated with the retarded chain dynamics.However,previous studies using differential scanning calorimetry(DSC)showed a weakened or even vanished melt memory in polymer nanocomposites,although adding nanoparticles in polymers often reduces chain mobility.In this work,we added two kinds of silica nanoparticles,O15 and W22 nanoparticles(with low and high surface silanol density,respectively),to poly(ε-caprolactone)(PCL),where the degree of chain entanglements near nanoparticles increases,and chain dynamics is gradually retarded with increasing silica content.The strongly aggregated W22 nanoparticles show significant heterogeneous nucleation,while well-dispersed O15 nanoparticles exhibit very weak heterogeneous nucleation.In contrast to the restrained self-nucleation effects during the rapid non-isothermal crystallization of two nanocomposites,we found evident melt memory effects during the slow isothermal crystallization in PCL/O15 nanocomposites as indicated by DSC and polarized light microscopy(PLM)measurements.The melt memory becomes stronger with more O15 nanoparticles,but depends non-monotonically on the loadings of W22 nanoparticles.The lifetime of the melt memory effects matches the chain re-entanglement time of the SN melts of nanocomposites,which is consistent with our previous work on PCL miscible blends.The complicated influence of silica on nucleation and crystal growth is induced by the spatial inhomogeneity of chain entanglements in the silica-filled PCL system.The decrease in the entanglement density of the interfacial chains in the SN melts of nanocomposites is conducive to nucleate under slow isothermal crystallization,but the chain mobility is still greatly restrained by the nanoparticles,causing the slow growth rate of spherulites and the difficulty to form nuclei under fast non-isothermal crystallization.

Further Understanding on the Three Domains of Isotactic Polypropylene by Investigating the Crystalline Morphologies Evolution after Treatment at Different Domains

The introduction of concept of the three domains of isotactic polypropylene（iPP） by Wittmann and Lotz et al. is an important advance in understanding the influence of the melt structures on the crystallization behaviors and consequent properties. To further understand the physical nature of the melt structures, the crystalline structures of i PP after thermal treatment in the three domains are systematically investigated. It is found that after treated at different domains the crystal morphologies, including the sizes and birefringence of spherulitic, the proportion of radial and tangential lamellae, etc., have distinctly different features. Our study reveals that the ＂nuclei＂ at domain II compose of locally ordered chains and the induced memory effect could not be erased under annealing treatment, while the ＂nuclei＂ at domain III compose of crystal fragments, which will aggregate under annealing process. Based on our results, highly schematic diagrams are proposed to illustrate the probable physical characteristics of the melt structures at the three different domains.