Semi-crystalline homopolymers

In the following section, we consider at some length the application of the technique to semi-crystalline homopolymers, including some discussion of experimental difficulties and the problems of finding suitable functions to represent the data. In subsequent section, we discuss in less detail the application of the technique to other classes of polymer. 

The observation of the broad line continuous wave (CW) spectrum of polyethylene (PE), and spectrum of polytetrafluoroethylene (PTFE) by 

Secondly, there is no firm basis for choosing the number of components to represent the lineshape. The observed lineshape is determined by the distribution of spins in the sample and their mobility, which may well show more complex behaviour than would be suggested from consideration of factors such as crystallinity, morphology, and microphase separation. In this regard, the investigator can only be guided by Occam s razor to take the least number of components that gives an adequate representation of the data, and by whether the behaviour of the components thus obtained correlates well with other properties of the sample. 

Since the lineshape is the Fourier transform of the FID, the information content of the two domains must be identical, and it should therefore be possible to analyse the on-resonance FID in a way analogous to the decomposition of the lineshape to yield equivalent information on molecular mobility. There are, however, some problems in making a direct comparison between the two sets of results. 

Firstly, the main source of instrumental distortion of the signal is quite different in the two domains. In the CW method, the signal is broadened due to the modulation of the applied RF, and corrections must be made for the effects of this broadening on the different components [30]. In the pulse method, the main distortion of the signal is due to the dead-time. Efforts may be made to minimise the dead-time by optimisation of instrumental characteristics, or in the case of rigid solids, some of the problem may be overcome by the application of suitable echo sequences [18, 37], but it cannot be totally eliminated. 

---

The true value of the chloropolymer (I) lies in its use as an intermediate for the synthesis of a wide variety of polytorgano-phosphazenes) as shown in Figure 1. The nature and size of the substituent attached to the phosphorus plays a dominant roll in determining the properties of the polyphosphazene. Homopolymers prepared from I, in which the R groups are the same or, if different, similar in molecular size, tend to be semi-crystalline thermoplastics. If two or more different substituents are introduced, the resulting polymers are generally amorphous elastomers. (See Figure 1.)... 

Microspheres were prepared from copolymers of DXO and l-LA and from homopolymer blends of DXO with l-LA and d,l-LA [129-131]. PLLA was partially miscible with PDXO and formed semi crystalline and dense microspheres. PDLLA and PDXO were fully miscible and formed homogeneous and amor-... 

Among the homopolymers, poly-XII showed smectic mesophases,poly-XIII and poly-NBDE were amorphous, and poly-COEN was semi-crystalline. The... 

Copolymers are ubiquitous and important because they allow monomers to be combined in such a way so as to provide useful and sometimes unique properties. For example, linear polyethylene (PE) and isotactic polypropylene (i-PP) are both semi-crystalline plastics, but copolymers of ethylene and propylene (EPR) (usually with other comonomers) are rubbers at room temperature (depending on composition). The homopolymers are shown in the top two figures in Figure 6-2, and if you don t know which one s which by now you should collapse in deepest humiliation. A section of an EPR copolymer chain is shown at the bottom. 

The second system was based on two semi-crystalline polymers polypropylene and polyamide. To attain potential reactivity, polypropylene was first grafted with maleic anhydride and the results of this radicalar melt-grafting are presented. The final blend was obtained in one-step extruding of the two homopolymers and the reactive polypropylene. 

In order to determine the effects of a given monomer on polyamide permeation properties, data obtained from copolymers where the monomer of interest was diluted by another diamine or diacid were often used. It is assumed that the OPV data for copolymers are weighted averages of the OPV data for the constituent homopolymers. The use of copolymers was necessitated by several reasons. It was often too difficult to form the homopolymer of interest with high enough molecular weight to allow formation of cohesive films. In other cases, the homopolymer was semi-crystalline, which, as will be described in the next paragraph, is undesirable for this study. 

As demonstrated before, the shifting involves three shift factors, one horizontal, usually expressed as aj, = b rip(T)/rip(Tp), where b = p T /pT is the hrst vertical shift factor that originates in the thermal expansion of the system (p is density). The subscript o indicates reference conditions, dehned by the selected reference temperature T, usually taken in the middle of the explored T-range. For homopolymer melts, as well as for amorphous resins, the two shift factors, aj, and b.j, are sufficient. However, for semi-crystalline polymers, a second vertical factor, v., has been found necessary — it accounts for variation of the crystallinity content during frequency scans at different temperatures [Ninomiya and Ferry, 1967 Dumoulin, 1988]. 

Tables in this chapter contain published pressure-volume-temperature data for amorphous homopolymers. Measurements below the melting temperatures for semi-crystalline materials are not included because of the potentially large variance among samples with differing degrees of crystallinity. Rogers [1] and Zoller [2] have also compiled equation-of-state data for amorphous polymers.

A more detailed interpretation of the internal structure of the PODMA domains is possible based on WAXS data for a semi-crystalhne block copolymer (Lam-20 nm) measured at room temperature using a Bruker D500 system (Fig. 12.8). Data for polystyrene and semi-crystalline PODMA homopolymers are shown for comparison. It is obvious that the scattering curve for the microphase-separated block copolymer can be interpreted as a superposition of contributions originating from both individual components. The first two peaks (1 32) at q 2.1 nm and q 4.3 nm indicate the nanophase separation and the lamellar packing of main and side chains in semi-crystalline... 

Depending on the molecular weight, PCL as a homopolymer has degradation times ranging from 2 to 4 years (Woodruff and Hutmacher, 2010) (Table 3.3). It is a semi-crystalline material, which means the degradation initiates in the amorphous regions of the polymer, while the crystalline phase initially will stay unaffected. As a result, the mechanical properties of the implant will not be influenced in a significant way. 

For polymer blends in which one component is crystalline the melting behaviour depends on circumstances. For immiscible blends, where the components are phase separated (prior to crystallisation) and act independently, the crystal melting temperature will be that of the homopolymer. In miscible blends, where the amorphous phase contains both components, the melting temperature will be lower than the equilibrium melting temperature for the crystallisable homopolymer, i.e. the crystalline polymer exhibits a melting point depression as discussed above. The Nishi and Wang approach (Sect 3.2) has been used to estimate the magnitude of the interaction parameters in a niunber of blends (Sect. 7). Poly(e-caprolactone) blends are often semi-crystalline and the above considerations, therefore, apply to many PCL blends. 

In semi-crystalline blends it should be noted that when the value of Tg of the amorphous phase is compared with values in Eqs. (21)-(23) the observed Tg reflects the composition of the residual amorphous phase rather than that of the overall mixture. Currently, the most common method of determining the crystalline content of a blend is to compare the energy associated with the melting endotherm, in, for example, a DSC thermogram, with that for the homopolymer and its previously estabfished crystalHne content. 

The principles involved will be discussed later in detail when the structure and morphology of semi-crystalline polymers are considered. For present purposes it suffices to state that the method has had reasonable success when appUed to homopolymers, although some important exceptions have been noted. However, major difficulties have been encountered when applied to random ethylene copolymers (68,69) as well as copolymers of isotactic poly (propylene). (70) It remains to be seen whether these methods can in fact be applied successfully to the other random copolymers. The examples that follow should be considered in this light. 

Solubility of Homopolymers Polyethylene (PE) is a semi-crystalline and nonpolar hydrocarbon polymer. Because of its apolar nature, this polymer is practically insoluble in SC-CO2 in conditions as harsh as 110°C and 2750 bar [4]. The interchange energy is then dominated by quadrupolar-quadrupolar interactions of CO2. To promote dipole-quadrupole interactions, the progressive introduction of fluorine atoms onto the polymer backbone has been studied through poly(vinyl fluoride) (—(CH2—CHF) —), poly(vinylidene fluoride) (—(CH2—CF2) —), and poly-tetrafluoroethylene (—(CF2—CF2) —) (see Figure 13.1). 

Copolymer type and melt flow rate also influenee the creep behaviour. Copolymer grades of PP have substantially lower ereep modulus than the homopolymer grades. PP has a similar modulus to high density PE, but its resistance to creep is much better and, at a equivalent time under similar load, the creep modulus of PP is more than that of high density PE. However, the ereep resistance of amorphous plastics is much better than the semi-crystalline plastics such as PP and PE. Creep resistance of PP could be further improved by addition of fillers or reinforcements. The creep behaviour of moulded artefacts is affected by the residual stress or orientation effect in the moulded article. 

---