agent of PP was divided into α- and β-nucleating agent. PP nucleated by α-nucleating agent formed α-modification (α-PP) [ 6 – 15 ] with good rigidity, while PP nucleated by β-nucleating agent might form β-modification (β-PP) with special properties [ 16
The nucleating efficiency and selectivity of different
β-nucleating agents was characterised and compared by differential scanning
calorimetry, (DSC) and temperature-modulated DSC (TMDSC). The nucleating agents
were the calcium salts of pimelic and suberic acid (Ca-pim and Ca-sub), linear trans-γ-quinacridone (LTQ), a commercial nucleator
NJ Star (NJS) and an experimental product (CGX-220). The efficiency and the
selectivity of Ca-sub and Ca-pim are extremely high. NJS is efficient above
a critical concentration, which is connected with its partial dissolution
in polypropylene melt. LTQ and CGX-220 possess strong overall nucleating ability
and moderate selectivity. Using TMDSC, we found that three consecutive processes
take place during the heating of β-nucleated samples cooled down to room
temperature: reversible partial melting of the β-form, irreversible βα-recrystallisation,
and the melting of the α-modification formed during βα-recrystallisation
or being present in samples prepared with non-selective β-nucleators.
Melting of the α-phase contains both reversible and irreversible components.
– 42 ]. Varga et al. [ 18 , 43 , 44 ] found that in the PP/PET blend melt-shearing, caused by fiber pulling, would form α -row-nuclei in situ, the surface of which may induce the growth of the β -modification of iPP resulting in a cylindrite of
In previous studies, it has been observed that cetyl palmitate molecules are arranged in an orthorhombic lamellar lattice structure ( Fig. 1 b), i.e., the most stable polymorphic form of this lipid is the β’ modification. During re
The characteristics of crystallization, melting and spherulitic growth of a random propylene copolymer (PRC) containing small amount of ethylene were studied in the presence of a selective Β-nucleating agent (calcium pimelate). It was established that the products of isothermal and non-isothermal crystallization are very rich in Β-modification but have mixed polymorphic composition. The formation of α-modification may be attributed to Βα-transition on the surface of growing Β-spherulites resulting in αΒ-twin-spherulites. During melting of PRC of Β-modification, the characteristics observed with Β-nucleated propylene homopolymers, namely, a Βα-recrystallization of recooled samples and separated melting of non-recooled samples (i.e. the melting memory effect), as well as a ΒΒ-recrystallization leading to a perfection of the structure within the Β-modification, are also demonstrated. The disturbance of regularity of the polymer chain highly reduces the tendency to Β-crystallization. In contrast to the observations with propylene homopolymers, the growth rate of α-modification (Gα) is higher than that of Β-modification (Gβ) and no critical crossover temperature can be found (T(Βα)=413 K) below whichGα>Gβ. The experimental results show that a partial disturbance of chain regularity by incorporation of comonomer units considerably reduces the tendency to Β-crystallization.
Ca salts of suberic (Ca-Sub) and pimelic acid (Ca-Pim) were synthesized and used as β-nucleating agents in different grades
of isotactic polypropylene (IPP). Propylene homo-, random- and block-copolymers containing these additives crystallize principally
in pure β-modification as demonstrated in isothermal and non-isothermal crystallization experiments. Ca-Sub proved the most
effective β-nucleating agent known, so far. It broadens the upper crystallization temperature range of pure β-IPP formation
up to 140C. The effect of the additives on the crystallization and melting characteristics of the polymers was studied. The
degree of crystallinity of the β-modification was found to be markedly higher than that of α-IPP. High temperature melting
peak broadening was first observed and discussed in literary results regarding the same phenomenon for α-IPP.
In this work the solid-state characterization of anhydrous D-mannitol has been performed: α and β modifications can be distinguished only by XRPD and FTIR as they show melting temperature
and enthalpy that are the same within the standard deviation. The understanding of the thermal behaviour of the δ form (obtained
by re-crystallization in acetone) has required XRPD experiments performed at variable temperature. This form during heating
undergoes a solid phase transition to α modification. By cooling a melted sample, under a wide range of experimental conditions,
a very fast crystallization occurs. Independently of the starting crystal form (β or δ form), the re-crystallization of D-mannitol from melt always leads to α form.