СТРУКТУРА И СВОЙСТВА
УДК 541.64:536.6
Isothermal Crystallization Behavior of PP and PP-g-GMA Copolymer at High Undercooling1
© 2013 г. G. M. Shashidhara, S. H. Kameshawari Devi, and S. Preethi
Department Polymer Science and Technology, Sri Jayachamarajendra College of Engineering, Mysore, 570006 India
e-mail: gm.shashi@yahoo.in (G.M. Shashidhara) Received June 13, 2012 Revised Manuscript Received November 29, 2012
Abstract — The study involves synthesis of polypropylene grafted with glycidyl methacrylate (PP-g-GMA) using three different initiators, benzoyl peroxide, dicumyl peroxide and tertiary butyl cumyl peroxide (TBSP). Among the peroxides used, dicumyl peroxide resulted in considerable reduction of molecular weight of the resulting graft copolymer. The melting/crystallization behavior and isothermal crystallization kinetics of PP homopolymer and PP-g-GMA copolymers were studied with differential scanning calorimetry (DSC) at high undercooling (44—60°C). The results showed that the degree of crystallinity and overall crystallization rate of copolymers is greater than that of virgin PP. Among the three initiators used, TBCP exhibited lowest half crystallization time. The isothermal crystallization kinetics of the PP and copolymers was described with the Avrami equation and Sestak-Berggren (SB) equation. The Avrami exponent n of the PP and copolymers were found to be in the range 1.03 to 1.41 at high undercooling conditions employed in this study. The agreement between the values of n calculated from SB kinetics and Avrami equation is satisfactory with few exceptions. The crystallization rate of PP-g-GMA copolymer was found to be more sensitive to temperature. The isothermally crystallized samples showed a single melting peak for PP while a double peak at lower temperature was recorded for PP-g-GMA copolymer samples. The equilibrium melting point Tm was deduced according to Hoffman-Weeks theory. The decrease of Tm recorded for the PP modified with GMA suggests that the thermodynamic stability of the PP crystals is influenced by the chemical interactions.
DOI: 10.7868/S0507547513060159
INTRODUCTION
Functionalized polymers are widely used as in situ compatibilizer in polymer blends. Glycidyl methacrylate (GMA) has been increasingly used as grafting monomer because of its epoxide function, which is highly electrophilic and capable of reacting with a variety of functional groups as carboxylic acids, amides and alcohols. Grafted polyolefins with polar groups are known to alter properties of the polymers, their crystallization characterization and morphology. PP-g- GMA has been used as a reactive compatibilizer in PP/PBT PP/SEBS-MAH, PP/CNBR, PP/PA1010, PP/PC, PP/PET, nylon6/liquid crystalline polymer blends and PP/clay nanocomposites [1—10].
Crystallization of polymers, which involves two consecutive phenomena — nucleation and growth, is governed by both thermodynamic and kinetic considerations. The heterogeneous nucleation takes place generally instantaneously upon cooling the polymer melt, but the number of effective nuclei is strongly temperature dependent. The temperature dependence of the effect of nucleating agent on crystallization becomes important since it helps to determine the processing conditions and controls the properties of molded products. Polymer crystallization is a subject
1 Статья печатается в представленном авторами виде.
of continuous interest and it has been studied extensively [4, 11, 12]. Much of the above work has concentrated on the measurement of overall rate of crystallization or the measurement of spherulitic growth rate of PP crystallization from melt at low and moderate
degrees of undercooling (AT = T® - Tc, where Tc is
crystallization temperature, T® is the equilibrium melting temperature), yet a detailed overall kinetics analysis at high undercooling (AT > 50°C), especially in the temperature range where the fastest crystallization rate would occur is still lacking. However, study of the behaviour of a semi crystalline polymer crystallizing in this temperature range is of great importance from practical view points. The practical importance is that such crystallization data may be very useful to polymer processing such as fiber spinning and injection molding, because most polymer shaping processes need a very fast crystallization rate to shorten the solidifying time. The importance also arises from the effect of final crystallinity on the physical and chemical properties of polymers.
In this study, the isothermal crystallization behaviour of PP and graft copolymer of PP and Glycidyl methacrylate (PP-g-GMA) at high undercooling conditions (the range of Tc is 100—116°C) is reported for the first time. The graft copolymer PP-g-GMA was
synthesized using three different initiators namely benzoyl peroxide (BPO), dicumyl peroxide (DCP) and tertiary butyl cumyl peroxide (TBCP). The isothermal crystallization kinetic studies of neat polypropylene (PP) and grafted PP were made using DSC. The model most often applied to isothermal crystallization data is the Avrami model [13—15]. In this study, another kinetic model, the Sestak-Berggren model [16], is applied to crystallization kinetics and the equivalency of the resultant kinetic parameters to those from the Avrami model is demonstrated.
EXPERIMENTAL
Materials
The polypropylene, REPOL H060MG - homo polymer (melt flow index 6 g/10 min) was supplied by Reliance industries limited, India. Reagent grade gly-cidyl methacrylate (GMA) (Sigma Aldrich) and styrene (Rolex chemical industries, India) were used without further purification. The peroxide initiators, benzoyl peroxide, dicumyl peroxide and tertiary butyl cumyl peroxide were procured from Qualigens, India, S.d. fine chemicals, India and Sigma Aldrich respectively.
Free Radical Grafting of GMA onto PP
The grafting reaction of GMA onto PP in melt phase was performed in an internal batch mixer (Polylab Rheomix RC 300P). Torque and temperature were monitored and recorded online. The required amount of PP was charged into the preheated mixing chamber. After stabilization of torque, GMA, styrene and BPO were introduced in a PP melt at 170°C and homogenized with 50 rpm. The mixing chamber was kept closed by a ram and mixing was continued for 10 min. After the reaction had completed (assessed by torque measurement), the samples were discharged from the mixing chamber and cooled to room temperature. The experiments were repeated under similar conditions using other initiators, DCP and TBCP. The graft copolymers prepared using the initiators BPO, DCP and TBCP are designated as PP-g-GMA(BPO), PP-g-GMA(DCP) and PP-g-GMA(TBCP) respectively. The amount of GMA grafted onto PP in copolymers was determined by titration. For this purpose 5 g of graft copolymer was dissolved in 25 ml xylene by re-fluxing. After complete dissolution, 100 ml of 0.05 N alcoholic KOH was added and further refluxed for one hour. The mixture was cooled and titrated against 0.05 N isopropanolic HCl using thymol blue as an indicator. The weight percent of GMA grafted onto PP was calculated using the relation
wt.% of GMA =
(V - V2 )
—)
1000/
W
x100, (1)
where V1 is the volume of alcoholic KOH (100 ml), V2 is the volume of 0.05 N HCl consumed and Wis the weight of graft copolymer dissolved.
FTIR Measurement
FTIR spectroscopic method was used to confirm grafting. The finely ground powder of copolymer sample and KBr was pelletized and the spectra were acquired using JASCO 4100 FTIR at a resolution of 4 cm-1. FTIR spectrum of pure PP was also obtained under similar conditions.
Melt Flow Index
The melt flow index (MFI) of PP and graft copolymers prepared in this work was determined in a standard melt flow indexer at 210°C and 2.16 kg weight in accordance with standard ASTM D 1238.
Differential Scanning Calorimetry
Typical DSC runs to obtain information about the crystallization and melting processes were performed at a heating rate of 10 grad/min under nitrogen atmosphere followed by a cooling cycle and reheating, using TA Instrument, model Q200 DSC. Isothermal crystallization from the melt was performed in the sample pan of the differential scanning calorimeter as follows: The sample (typically 5 mg) was heated at 20 grad/min to 190°C and maintained isothermal conditions for five minutes to eliminate any previous thermal history. The resulting melt was rapidly cooled at 80 grad/min to the predetermined crystallization temperatures (Tc) in the range 100-116°C and maintained at Tc for the time necessary for isothermal crystallization. For each material, a minimum of four crystallization experiments were performed over a range of temperatures. The exothermal curves of heat flow as a function of time were recorded. The kinetic analysis of the crystallization process was carried out using thermal analysis advantage software supplied by TA instruments.
RESULTS AND DISCUSSION
Effect of Type of Initiator on Degree of Grafting
The initiator plays a major role and must be well chosen in order to achieve optimal reaction efficiency. Half-life data of initiators can be used for comparison purposes to select among different peroxides. Half-life of peroxide is defined as the time it takes for one half of a given quantity of peroxide in dilute solution to decompose at a given temperature. For convenience in comparing the stability of peroxides in dilute solutions, peroxides are commonly listed according to the temperatures at which they have half-lives of10 or 1 h. The values of half life temperature of the three peroxides used in the present work are given in Table 1. The
Table 1. Characteristic properties of peroxides used and copolymers synthesized
Peroxide Half life temperature, °C Polymer/copolymer GMA content, wt. % MFI, g/10 min
10 h 1 h
BPO 72.9 91.7 PP-g-GMA(BPO) 11.20 14
DCP 117.1 137.0 PP-g-GMA(DCP) 10.56 110
TBCP 123.6 143.6 PP-g-GMA(TBCP) 10.86 70
- - - PP - 10.6
Table 2. Characteristic
Для дальнейшего прочтения статьи необходимо приобрести полный текст. Статьи высылаются в формате PDF на указанную при оплате почту. Время доставки составляет менее 10 минут. Стоимость одной статьи — 150 рублей.