A comparison between the morphology of semicrystalline polymer blends of poly(ε-caprolactone)/poly(vinyl methyl ether) and poly(ε-caprolactone)/(styrene-acrylonitrile)

The morphology of polymer blends of poly(ε-caprolactone) (PCL) and poly(vinyl methyl ether) (PVME) is compared with that of PCL and a random copolymer of styrene and acrylonitrile (SAN). The main objective is to determine the influence of the glass transition temperature of the amorphous component (Tg,a) on the morphology of the semicrystalline polymer blends. These blends represent the two extreme cases corresponding to Tc < Tg,a and Tc > Tg,a, where Tc is the crystallization temperature. The morphology of these blends, with PVME and SAN representing the amorphous components, have been studied by small angle X-ray scattering. For both blends the long period increases with the addition of amorphous polymer, which is a strong indication for an interlamellar morphology. D.s.c. experiments, including enthalpy relaxation, are used to investigate the crystallinity and the interphases. The overall amount of crystallinity in both blends decreases with increasing content of amorphous polymer. However, the fraction of PCL that crystallizes decreases in PCL/SAN and increases slightly in PCL/PVME. Apparently, the addition of the low Tg,a PVME improves the crystallization of PCL in accordance with a simple Gamblers Ruin Model type argument. The high Tg,a of SAN means this does not occur in PCL/SAN blends. Conventional d.s.c. experiments show an interphase of pure amorphous PCL in PCL/SAN blends and enthalpy relaxation experiments demonstrate its presence in PCL/PVME blends as well.

Machinability Of Polyamide 6 Under Cryogenic Cooling Conditions

Abstract Machining of polymeric materials can attain interest when the production lot does not justify the cost of molds or extrusion dies, or when the product to be manufactured requires dimensional accuracy not achievable otherwise. In this framework, the present study aims at evaluating the machinability of the polyamide 6 as a function of the cooling conditions. Two different cryogenic cooling configurations were adopted, whereas the conventional flood cooling was used as reference for sake of comparison, leading to machining conditions under very different temperature ranges. The polyamide 6 machinability was evaluated in terms of surface integrity (surface roughness, surface defects, crystallinity percentage and hardness) and chip morphology. Results show that the polyamide 6 has to be cut in a specific temperature range, namely between -20°C to 20°C, in order to get the best surface finish, namely achieving the lowest surface roughness and density of defects. In addition, the cryogenic cooling is proved to produce harder surfaces than the flood condition, but leaving unaltered the polymer crystallinity degree

Enthalpy relaxation, crystal nucleation and crystal growth of biobased poly(butylene isophthalate)

The crystallization behavior of fully biobased poly(butylene isophthalate) (PBI) has been investigated using calorimetric and microscopic techniques. PBI is an extremely slow crystallizing polymer that leads, after melt-crystallization, to the formation of lamellar crystals and rather large spherulites, due to the low nuclei density. Based upon quantitative analysis of the crystal-nucleation behavior at low temperatures near the glass transition, using Tammann’s two-stage nuclei development method, a nucleation pathway for an acceleration of the crystallization process and for tailoring the semicrystalline morphology is provided. Low-temperature annealing close to the glass transition temperature (Tg) leads to the formation of crystal nuclei, which grow to crystals at higher temperatures, and yield a much finer spherulitic superstructure, as obtained after direct melt-crystallization. Similarly to other slowly crystallizing polymers like poly(ethylene terephthalate) or poly(l-lactic acid), low-temperature crystal-nuclei formation at a timescale of hours/days is still too slow to allow non-spherulitic crystallization. The interplay between glass relaxation and crystal nucleation at temperatures slightly below Tg is discussed

Novel aspects in the crystallization of polybutene-1

Semicrystalline polymers cover over two thirds of commercially produced polymeric materials, and have been widely applied to many areas of the modern society, including building and construction, electronics, packaging, etc. Understanding the behavior of polymer crystallization is of critical importance due to the significant impact of the crystallization process on the properties of materials. Polymorphism is the ability of a polymer, in analogy with low molecular mass substances, to crystallize in different modifications, characterized by different crystal structures (polymorphic forms). Much effort has been made to find proper methods to develop different crystal modifications for polymorphic polymers. However, it is still a challenge to control the polymorphic outcome of the crystallization process. Furthermore, semicrystalline polymers are composed of stacked crystalline lamellae and entangled amorphous polymeric chain segments in between them. The interaction between amorphous and crystalline phases plays an important role in determining final mechanical and transport properties. Despite this importance, the effect of polymorphism on the amorphous phase is not well clarified because of its complexity. In this thesis, one typical polymorphic polymer, polybutene-1 (PB-1), was selected for a detailed crystallization study. Polybutene-1 is one of the most investigated polymorphic polymers. It has applications in pipes and films with a service life up to 50~100 years due to its excellent mechanical properties. In practice, among all crystalline polymorphisms within PB-1, Forms I and II are the most relevant modifications from the processing perspective. With the goal to establish a comprehensive understanding of the heterogeneous nucleation between these two modifications (cross-nucleation), we monitor the crystallization process of Form II induced by Form I crystals with different type of substrate (spherulitic, hedritic, fiber-like) using a direct investigation technique of optical microscopy. The different cross-nucleation efficiencies of Form II are tentatively attributed to differences in the Form I lamellar thickness, on the basis of an epitaxial crystallization and secondary nucleation mechanism. A quantitative analysis of the induction time for nucleation determined the cross-nucleation energy barrier, which could be reasonably described by classical models. The results revealed that the rate determining step for nucleation is the growth of the nucleus to critical sizes. Furthermore, the hypothesized epitaxy in PB-1 Form II-on-Form I cross-nucleation is probed by employing in-situ nanofocused synchrotron X-ray diffraction. Comparing the two-dimensional diffraction patterns at the interface between the two modifications, a preferred mutual orientation of the two structure, with the (200)II plane aligned ~8.5\uba apart from the (110)I plane, is revealed. This demonstrates a parallel (110) plane between the two polymorphs. Then, both mismatches between the inter-chain distances and along the chain axes within (110) plane were considered, and found to lay well-below the accepted mismatch criterium for epitaxy. This confirms that the cross-nucleation of Form II on Form I occurs at the (110) contact planes through epitaxial nucleation. Next, an in-depth study of fiber-induced nucleation ability and crystalline morphology in polybutene-1/single fiber composites is presented. Using different fibers as substrates, we could unveil the difference of Form II crystalline morphology: a transcrystalline layer (TCL) induced by PB-1 Form I fiber and hybrid shish\u2013calabash structure (HSC) induced by other fibers, namely carbon, glass, PP, PLLA homocrystal and stereocomplex. Based on a quantitative analysis of the nucleation kinetics, it was found that the nucleation free energy barrier is affected both by surface roughness and surface chemistry or specific surface-polymer interactions (such as epitaxy). In view of the number of nucleation sites correlating with the fiber surface roughness, it was demonstrated that transcrystallinity can be obtained only when a sufficient amount of nucleation sites is available, notwithstanding the height of the nucleation barrier. Besides the phenomenon of heterogeneous nucleation, the three-phase structure is also influenced by crystal polymorphism. Therefore, in the last part of the thesis, we focused on the study of the rigid amorphous fraction in both polymorphs, i.e., the part of amorphous chains constrained by direct coupling to the crystalline lamellae. Isochronous aging experiments with differential scanning calorimetry (DSC) on both crystalline phases are performed, in a wide temperature range between the glass transition of the mobile (bulk) amorphous fraction and the onset of crystal melting. An endothermic peak above the aging temperature is typically observed. The trend of the enthalpy of this annealing peak with temperature can be described by a bell-shaped curve, approaching zero recovered enthalpy at temperatures of 100-110 \ub0C, and 40-50 \ub0C for Form I and Form II, respectively. These temperatures are thus identified as the upper limit of the glass transition of rigid amorphous fraction for the two polymorphs. Overall, our results demonstrate that for PB-1, at least within the investigated temperature range, the annealing peaks can be related to the RAF strive to attain thermodynamic equilibrium in its glassy state

Fingerprints of homogeneous nucleation and crystal growth in polyamide 66 as studied by combined infrared spectroscopy and fast scanning chip calorimetry

Homogenous crystal nucleation and growth in polyamide 66 (PA66) are followed in situ by means of a combination of FTIR spectroscopy and fast scanning chip calorimetry (FSC). Therefore, a novel setup with a calorimetry chip equipped with an IR-transparent SiN membrane was developed, which enables to examine IR spectroscopic and FSC experiments on the identical specimen. Because of the small amount of sample material (~ 100 ng), it is possible to achieve heating and cooling rates up to 5000 Ks−1, and hence to quench the sample into a fully amorphous state without quenched-in homogeneous crystal nuclei. Annealing the film then allows to determine the onset of homogenous nucleation and crystal growth by means of FSC, whereas molecular interactions are unraveled by FTIR spectroscopy. It is demonstrated that different moieties of PA66 respond distinctly during crystallization; far-reaching interactions such as hydrogen bonding are established prior to onset of short-range steric hindrance

Physical Aging Behavior of a Glassy Polyether

[Abstract] The present work aims to provide insights on recent findings indicating the presence of multiple equilibration mechanisms in physical aging of glasses. To this aim, we have investigated a glass forming polyether, poly(1-4 cyclohexane di-methanol) (PCDM), by following the evolution of the enthalpic state during physical aging by fast scanning calorimetry (FSC). The main results of our study indicate that physical aging persists at temperatures way below the glass transition temperature and, in a narrow temperature range, is characterized by a two steps evolution of the enthalpic state. Altogether, our results indicate that the simple old-standing view of physical aging as triggered by the α relaxation does not hold true when aging is carried out deep in the glassy state.D.C. research was funded by MICINN-Spain and FEDER-UE, grant PGC2018-094548-B-I00; and the Basque Government, grant IT-1175-19. J.M. thanks MICINN /FEDER for grant Ref. PGC2018-094620-A-I00Eusko Jaurlaritza; IT-1175-1

Polymer nucleation and crystallization investigated by in situ combination of atomic force microscopy (AFM) and fast scanning calorimetry (FSC)

A chip based fast scanning calorimeter (FSC) is used as a fast hot-stage in an atomic force microscope (AFM). This way, the morphology of materials with a resolution from micro to nano meters after fast thermal treatments becomes accessible. FSC provides crystallization/melting curves of the sample just imaged by AFM. A combined AFM-FSC device is described, where the AFM sample holder is replaced by the FSC chip-sensor. The sample can be repeatedly annealed at pre-defined temperatures and times and the AFM images can be taken from exactly the same spot of the sample

Cellulose Reinforced Thermoplastic Composites By In-Situ Ring-Opening Polymerization

Over the past two decades, the increasing concern about the negative environmental impacts of synthetic materials has led to rising interests in utilizing renewable natural resources to develop polymer materials with comparable properties and performance to their synthetic counterparts. One of the major fields of interest is polymer composites where the replacement of synthetic fibers with bio renewable natural fibers is of great potential. However, the processing difficulties, in terms of fiber dispersion and thermal stability have limited the application of cellulosic fibers to polymers with low processing temperatures which are mostly hydrophobic polymers. As a result, the true reinforcing ability of the fiber could not be fully exploited due to polymer-fiber incompatibility. This dissertation discusses a novel approach to develop nanocomposite and composite materials based on high melting point polyamide 6 engineering thermoplastic matrix utilizing the in-situ ring-opening polymerization Both nanoscale cellulose nanocrystals as well as macroscale natural fibers were used as reinforcement. The initial study consisted of a detailed analysis of physical, viscoelastic and rheological properties polyamide 6 nanocomposites reinforced with cellulose nanocrystals in correlation with the morphology and microstructure of the nanocomposites. These nanocomposites were then used a masterbatch for further processing via melt extrusion technique. The effect of surface modification of cellulose nanocrystals with silane coupling agents on isothermal and non-isothermal crystallization of the obtained nanocomposites were fully investigated using a number of different theoretical models to gain a better understanding of the interrelation of surface functionality, microstructure and crystallization behavior. In addition, the effect of polymer-particle interfacial modification on shear and extensional rheological behavior as well as the mechanical properties of the nanocomposites were investigated. The results were correlated with the development of “interphase” in modified systems as confirmed by quantitative nanomechanical analysis. In addition, a series of polyamide 6 composites reinforced with flax fabric and kraft pulp cellulose fibers were successfully developed using vacuum assisted resin infusion process and a through processing-structure-property relationship study was conducted. The findings of this research effort provide a better understanding of the complex processing-structure-property relations of engineering thermoplastics reinforced with cellulosic fibers