We successfully developed an effective strategy for preparing poly(butylene succinate) (PBS) material with exceptionally high crystallinity (94.4% from density data) and melting point (∼136 °C) from its urea inclusion complex under normal air pressure for the first time. The strategy consists of three steps: co-electrospinning PBS with urea, thermal treatment, and coalescence of PBS in water. Electrospinning provides well physical mixing between PBS and urea. Thermal treatment causes the assembly of PBS and urea and leads to formation of their inclusion complex, in which PBS chains are separately isolated in urea channels. During the coalescing process the isolated and ordered PBS chains formed crystals with lamellar thickness larger than 26 nm—larger than 5-fold of common PBS crystal—after removing host urea frames. PBS crystals obtained with such high crystallinity and melting point are proved to be the extended-chain crystals (ECCs). Comparative study reveals that the electrospinning and thermal treatment process are necessary in this strategy for preparation of ECC although many other methods can be also used to obtain inclusion complex. Further study shows that the strategy is also feasible for obtaining ECCs of other aliphatic polyesters.

Terahertz time-domain spectroscopy (THz-TDS), which has been proved to show promising application in complex polymer systems, was employed to investigate the polymorphism phenomenon and crystal transformation of polylactide (PLA) in this study. The THz-TDS shows sensitive response on the crystal structure. The α'-form, α-form and stereocomplex crystals exhibit absorption peaks of lattice vibration at 1.82, 2.01 and 2.09 THz, respectively. THz-TDS has no direct chirality identification on the difference between poly (d-lactide) (PDLA) and poly(l-lactide) (PLLA). However, the PLA stereocomplex shows an extra and distinctive absorption peak at 1.43 THz compared with homo-PLA, and the peak was proved to be stemmed from the collective vibration of L-lactic unit and D-lactic unit pairs connecting by hydrogen bonds. This is the first time that THz-TDS has been proved to be of great potential in identification of polymer stereocomplex crystal. Also, the α'→α crystal transformation of PLA were intuitively investigated at 120 °C using THz-TDS, while the transformation rate was quite slow.

Herein, we demonstrated that poly(ethylene oxide) (PEO), urea and thiourea can crystallize into novel ternary complex with the molar ratio of guest polymer and host small molecule as 3:2, and proved that the ternary complex behaves isomorphism phenomenon by varying the ratio between urea and thiourea for the first time. This observation gives a boost to prepare co-crystals of different small molecules that cannot be obtained by direct mixing without the aid of polymer chains.Download high-res image (151KB)Download full-size imageWe demonstrate that urea, thiourea and poly(ethylene oxide) can show isomorphism phenomenon in their ternary comoplex for the first time. The molecular level hybrid crystal lattice between urea and thiourea can be realized with the aid of poly(ethylene oxide).

•PHSF is employed to explore the effect of intersegmental interaction in crystalline lattice on the polymorphism.•PHSF shows HF content-dependent polymorphism.•PHSF shows crystallization temperature-dependent polymorphism.•The intersegmental hydrogen bonding interaction is the crucial contribution to orthorhombic crystal.•Orthorhombic crystal is more thermal stable than monoclinic crystal.Poly(hexamethylene succinate-co-hexamethylene fumarate)s (PHSF) with low hexamethylene fumarate content (CHF, between 0 and 18 mol%) were prepared and employed to explore the effect of intersegmental interaction in crystalline lattice on the polymorphism phenomenon. Wide angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR) results demonstrate that the crystal modification of PHSF exhibits not only CHF-dependent but also crystallization temperature (Tc) dependent. Both high CHF and high Tc benefit the formation of orthorhombic modification, while low CHF and low Tc benefit the formation of monoclinic modification. More intriguingly, the specific “CCCO⋯HCC” hydrogen bonds formed between fumaryl units provide a direct way to regulate the polymorphism from perspective inside chain structure. When CHF is between 8 and 12 mol%, the crystal modification of PHSF is determined by formation of intersegmental hydrogen bonding interaction or not through varying Tc. At high Tc, the specific hydrogen bonds form and render the formation of orthorhombic crystal; while at low Tc, only van der Waals forces remain and PHSF chains crystallize into monoclinic crystal. The content of carbonyl group belonging to fumaryl in polyester chain is a key factor for the formation of hydrogen bonds and the selection of crystal structure during crystallization. Additionally, the orthorhombic crystal shows higher equilibrium melting point than monoclinic crystal; and the monoclinic crystal can transform to orthorhombic crystal by annealing, revealing that orthorhombic crystal is more thermally stable.

Poly(butylene adipate-co-butylene fumarate) (PBAF) copolyesters with different butylene fumarate (BF) contents were synthesized, and the effect of BF units on the polymorphism, crystallization behavior and crystal transformation were studied using various methods. BF units are found to cocrystallize into both α- and β-form crystal lattices, and the β-form lattice shows a better accommodation degree for BF units than the α-form. Incorporation of BF units alters the formation temperature ranges for polymorphism behavior, and facilitates the β-form crystal at high temperature due to their stronger restriction effect on primary and secondary nucleation of the α-form crystal than the β-form crystal. Furthermore, BF units retard β to α solid–solid crystal transformation, and a novel triple melting phenomenon emerges in the DSC curve of β-form PBAF, which is reasonably attributed to the melting of the original β-form crystal and two types of newly-formed α-form crystals during heating. The β to α transformation takes place through both solid–solid and solid–liquid–solid (melt–recrystallization) routes. A special hydrogen-bonding interaction between neighbouring chain stems in the crystal phase is suggested to be responsible for the restriction of the solid–solid route and the induction of the solid–liquid–solid route for β to α transformation.

•PVA porous scaffold with optimized mechanical properties and release capability was prepared.•The tensile strengths of PVA scaffold were 9.3 and 1.48 MPa in dry and hydrogel states, respectively.•The final cumulative release of ciprofloxacin could reach as high as 89.3%.•The reasons for PEG enhancement on the ciprofloxacin release efficiency was in detail studied.Hydrophilic porous polymer scaffolds have shown great application in drug controlled release, while their mechanical properties and release efficiency still need further improvement. In the current study, the porous scaffolds of polyvinyl alcohol (PVA) prepared by quenching in liquid nitrogen and freeze drying method from different original concentration aqueous solutions were fabricated. Among different PVA scaffolds, the scaffold stemming from 18 wt.% PVA aqueous solution exhibited the best mechanical properties, 10.5 and 1.54 MPa tensile strengths for the dry and hydrogel states respectively. The inner morphology of such PVA scaffold was unidirectional honeycomb-like structure with average microchannel section of 0.5 μm, and the scaffold showed porosity of 71% and rather low ciprofloxacin (Cip) release efficiency of 54.5%. Then poly(ethylene glycol) (PEG) was incorporated to enhance the Cip release efficiency. The release efficiency reached 89.3% after introducing 10 wt.% PEG, and the mechanical properties of scaffold decreased slightly. Various characterization methods demonstrated that, adding PEG could help to enlarge the microchannel, create extra holes on the channel walls, weaken the interaction between PVA chains and Cip, and miniaturize the crystal size of Cip. All these effects benefit the dissolution and diffusion of Cip from scaffold, increasing its release capability. Moreover, based on biocompatible material composition, PVA/PEG scaffold is a non-cytotoxicity and have been verified that it can promote cell growth. And PVA/PEG scaffolds loaded with Cip can completely inhibit the growth of microorganism because of Cip sustaining release. The PVA scaffold would have a good potential application in tissue engineering, demanding high strength and well drug release capability.

Poly(L-lactide) (PLLA)/pristine vermiculite nanocomposites were prepared by melt blending in a twin-screw extruder, and the detailed information of vermiculite dispersion state and effect of vermiculite on thermal and mechanical properties were systematically studied. The results show that the dispersion of vermiculite in the matrix is quite well when the loading content does not exceed 3 wt%. Pristine vermiculite can obviously improve the melt-crystallization temperature during the nonisothermal crystallization. Both crystallization time span and spherulitic size of PLLA decrease with the increasing amount of vermiculite under isothermal crystallization condition by enhancing the primary nucleation of PLLA. And the adding vermiculite can also improve the tensile modulus and Izod impact strength of PLLA. The intrinsic mechanism for the nucleating effect of vermiculite on PLLA is proposed to be the epitaxial crystallization and specific interaction between vermiculite and PLLA.

Biodegradable middle-chain-length polyhydroxyalkanoates (mcl-PHAs) have been developed as promising materials in various fields, while their crystalline structure and crystallization behavior which are closely related to application still need advanced study. Here, poly(R-3-hydroxydecanoate) (PHD) was employed as model polymer for in-depth study of crystallization behavior and phase transition of mcl-PHAs. The results confirmed formation of layered crystalline structure and hydrogen bonds in phase II. During formation of phase II, main- and side-chains crystallize simultaneously, while the latter component shows apparently faster rate at the beginning stage of crystallization. When melt quenched to low temperature, PHD forms phase I which contains crystalline side-chains and arrangement-inhibited main-chains. And phase I can transform into phase II through a solid-solid transformation process.

Nanocomposites formed between poly(1,4-butylene adipate) (PBA) and carbon nanofibers (CNF) were prepared and characterized. The results showed that CNF had significant α-form crystal nucleating ability on PBA, and the α-form crystals could appear at rather low crystallization temperatures in PBA/CNF composites, for example 0 °C. Furthermore, a novel plateau content phenomenon of α-form crystals was discovered, and the value of the plateau content almost increased linearly with increasing CNF loading content. Microscopic morphology data suggested that PBA chains could wrap (or coat) around the nanofillers after the solution mixing process to form an interaction region. And in situ FTIR spectra proved the existence of multiple-weak CH–π interactions between CH2 groups on PBA chains and sp2 structures on the surface of CNF in composites, which helped to induce some precursory structures in the interaction region containing a chain conformation close to that in α-form crystals. Thus, the wrapping (or coating) behavior and precursory structures are responsible to the formation of α-form crystals at low temperature and the appearance of a novel plateau content phenomenon of α-form crystals. The change of PBA polymorphism behavior in nanocomposites provides a pathway for evaluating the interaction between polymer chains and nanofillers more intuitively.

•A new PEO–thiourea complex with EO:thiourea molar ratio of 4:1 was demonstrated.•The new complex was suggested to adopt orthorhombic modification.•The complex probably adopts channel structure with two parallel PEO chains inside.•Study on the stability of complex was also carried out.Study on the structure of PEO–thiourea binary system has been carried out by several groups, but there still remain some questions. In this study, PEO–thiourea complexes with different thiourea fractions were prepared through freeze-drying and characterized by differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD), Fourier transform infrared (FTIR) microscopy and Raman scattering. A new PEO–thiourea complex with EO:thiourea molar ratio of 4:1 (form II) was demonstrated for the first time. Then the phase diagram of PEO–thiourea binary system was improved, containing two particular complexes at thiourea molar fraction of 0.4 (form I) and 0.2 (form II), respectively. Further analysis suggested that PEO–thiourea form II complex adopts orthorhombic unit cell, and probably takes channel structure with two side-by-side parallel PEO chains incorporated inside the thiourea clathrate. The form II complex is quite stable in pure state at room temperature; while gradually convert into from I by solid–solid transformation when a few form I complex serves as nucleator.

Stretch-induced structural changes of poly(butylene adipate) at various temperatures were systematically investigated by X-ray diffraction method. It was found that tensile deformation could lead to both occurrence of α to β and β to α crystal transition via solid–solid process. Experimental results shows that either α or β form can be adopted as the stable phase for PBA crystal depending on the competition between tensile strain and temperature. Higher temperature favors β to α crystal transformation, while larger strain would result in α to β crystal transformation. Due to the conversion of crystal stability during stretching, a unique “β to α to β” transformation phenomenon of PBA appears at appropriate temperature for original β spline. This study provides evidence that both temperature and tensile strain can alter the relative stability of polymorphic crystals of semi-crystalline polymers.

•Random copolyetsers formed between hexamethylene succinate and hexamethylene fumarate were found to show as strict isomorphism in limited composition range.•A special hydrogen-bond interaction in “CC–H⋯OC–CC” form was found in PHF and PHSF.•Crystal modification changes from monoclinic to orthorhombic after introducing hexamethylene fumarate into PHS.•The conformation of hexamethylene segments in crystalline phase changes from all-trans to gauche after introducing hexamethylene fumarate.A series of random poly(hexamethylene succinate-co-hexamethylene fumarate) (PHSF) samples was synthesized through a two-step reaction process, and their properties were systematically studied using wide angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), and Fourier transform infrared spectroscopy (FTIR). The crystal modification of polyester changes from monoclinic to orthorhombic after substituting hexamethylene succinate (HS) units by hexamethylene fumarate (HF) units, and the chain conformation changes from all-trans to gauche. The copolyesters were found to be in a particular case as strict isomorphism in a limited composition range, adopting the same crystal modification when HF content ranged from 19 to 100 mol%. The melting point of the copolyester increases almost linearly with increasing molar content of HF, and the melting enthalpy hardly changes. Interestingly, FTIR results suggest that a special hydrogen-bonding interaction, in “CC–H···OC–CC” form, forms in the crystalline phase of PHSF and poly(hexamethylene fumarate) (PHF). The strong hydrogen-bonding interaction might be responsible for the transformation of conformation from all-trans to gauche in hexamethylene segments, and triggers the variation of crystal modification.

•The PEO film contains a ring at the edge and a volcano-like profile in the middle.•A special ring-like nucleation provides a visual mark of Marangoni convection.•The decreasing viscous stress is the key point of contact line recession.The drying processes of poly(ethylene oxide) (PEO) droplets on isothermal and non-isothermal substrates were studied and interesting results were found. When PEO droplet was evaporated isothermally at 30 °C, common evaporation with usual ‘puddle’ structure was formed. However, at a heating rate of 20 °C/min, the drying process developed two stages. At lower temperatures, a pinned contact line drying behavior was observed. At elevated temperatures (e.g., >60 °C), the free liquid surface started to shrink. The resultant PEO film appeared as a ‘dual-ring’ profile with a ring-like nucleation. Further annealing and AFM results suggested that nucleation behavior was due to the PEO chain memory effect of flow field in the drying droplet. This memory effect provided an inner and visual mark of the flow field. The rheological properties and surface temperature were also investigated to estimate Marangoni number (Ma), which increased significantly with temperature. The following mechanism was therefore proposed. At relatively low temperatures, the capillary flow was dominant, resulting in the formation of ‘ring’ at the edge. When the temperature increased to 60 °C, high temperature caused a sharp decrease of viscous stress and the increase of surface stress gradient, which resulted in the receding of contact line. Consequently, Marangoni convection became dominant and ‘dual-ring’ morphology finally formed.Marangoni convection is the dominant flow when the substrate temperature is above 60 °C, and the peculiar ‘dual-ring’ profile of the PEO film contains ring-like profile at the edge and volcano-like profile in the middle. A ring-like nucleation distribution shows that the memory effect of polymer crystallization behavior on flow field could provide a visual mark for Marangoni convection, and lamellar vortex structure in the spherulite center further confirms this conclusion. Marangoni convection is the dominant flow in the later high temperature drying stage, and results in peculiar ‘dual-ring’ profile and special ring-like nucleation of the deposited PEO film.

Inclusion complexes formed from host small molecules and guest polymers have provided a novel platform to study both the physical properties of isolated polymer chains and crystallization behavior of host molecules. In this report, we have studied the crystalline structure, melting behavior and infrared information of inclusion complexes formed between polyethylene glycol oligomers and urea molecules. The effect of molecular weight and terminal group of guest PEG oligomer on the melting behavior of inclusion complex is presented. FTIR spectra reveal that terminal –OH of PEG is hydrogen-bonded to urea in inclusion complex, while the terminal –OCH3 is free. The melting behaviors of complexes are interpreted using either Gibbs–Thomson equation for lamellar crystals with definite channel length or Flory equation for crystals with infinite channel length and included defects. The results show that the interaction between terminal –OH in PEG chain and urea induces larger interfacial free energy or stronger defect effect, compared to the terminal –OCH3 in the inclusion complexes, which results in the particular order of melting point: Tm(DPEG500UIC) > Tm(MPEG550UIC) > Tm(HPEG600UIC).