Method for determining branch distribution of polymer

A branch distribution of a crystalline polymer is determined by melting the polymer at a temperature sufficiently higher than the melting point thereof, then cooling the melted polymer to room temperature in such a manner that isothermal crystallization of the polymer is allowed to take place stepwise at intervals of 3.degree. C. or higher at least from near the upper limit of the melting point of the polymer, then raising the temperature to melt the crystals and determining a branch distribution from the melt behavior, using a thermal analyzer.

BACKGROUND OF THE INVENTION 
The present invention relates to a method for determining a branch 
distribution of a crystalline polymer and more particularly to a method 
for determining a branch distribution of a crystalline polymer on the 
basis of a melt behavior of the polymer in which the polymer is stepwise 
crystallized isothermally and thereafter melted, using a thermal analyzer. 
PRIOR ART 
It is known that in the case of a polymer containing a branch distribution, 
particularly a polyolefin resin or a composition thereof, the branch 
distribution thereof exerts a great influence on product physical 
properties. Especially, in the case of linear low-density polyethylene 
(LLDPE), its branch distribution exerts a serious influence on the heat 
selability or transparency of films formed therefrom or on the stress 
cracking resistance of pipes or the like formed therefrom, so various 
methods for determining a branch distribution are now under study. 
According to a branch distribution determining method which is used at 
present, each of components obtained by molecular weight fractionation is 
determined for an average branch density using NMR or an infrared 
spectrophotometer. By this method, however, only a branch density in each 
of certain molecular weight ranges can be determined, and no information 
is provided about the branch distribution in the whole of a composition. 
As a branch distribution determining method which has solved the 
above-mentioned problem there is known a temperature rising elution 
fractionation (TREF) method. This method utilizes the fact that the 
solubility of a crystalline polymer in a solvent depends on a branch 
density, and this method has been studied as a method for structural 
analysis of LLDPE. A more detailed structural analysis is also being made 
by the combination of the foregoing molecular weight fractionation and 
TREF method. 
However, the TREF method requires special apparatus and skill, and a long 
time of 15 to 20 hours is required for the determination, including 
pretreatment, so it is difficult to apply the TREF method to the quality 
control in a commercial plant for example. 
On the other hand, Paul J. Flory has published a theory of equilibrium 
melting point drop based on a copolymer composition [Trans. Farad. Soc:, 
51, 848 (1955)]. According to this theory, as the density of amorphous 
units contained in a crystalline polymer chain increases, an equilibrium 
melting point drops linearly. The "amorphous units" as referred to herein 
indicate short-chain branches formed by an .alpha.-olefin copolymerized 
with ethylene. According to this theory, if a crystalline polymer is 
crystallized stepwise thermodynamically under conditions close to 
equilibrium crystallization, it can be crystallized (segregation) at each 
of various branch densities. And if the crystalline polymer thus 
crystallized is melted stepwise, there ought to be obtained a melting heat 
chart corresponding to the branch densities. It is expected that by using 
the said chart there will be obtained information on a branch 
distribution. 
It is reported in literatures [see, for example, Journal of Thermal 
Analysis, Vol.10, pp.433-440 (1976)] that if a crystalline polymer 
containing a branch structure is crystallized and then melted while the 
temperature is dropped stepwise using a thermal analyzer, the process of 
the crystallization is memorized as it is and is reflected in the melt 
behavior. It is presumed that the cause of this phenomenon has a bearing 
on the branch distribution. 
Thus, although it is suggested that the branch distribution of a 
crystalline polymer and the crystallization or the melt behavior have a 
close relation to each other, a method of clarifying the relation of the 
two and utilizing it concretely in the analysis of branch distribution has 
not been known. 
SUMMARY OF THE INVENTION 
It is the object of the present invention to provide a method for 
determining a branch distribution of a polymer in a simple manner, in a 
short time, with an accuracy equal to that of the TREP method, using the 
Flory theory and without using any special apparatus. 
Having made extensive studies with respect to the above-mentioned object, 
the present inventors have discovered that a correlation of high accuracy 
is obtained between the melt behavior and the branch distribution of a 
polymer by isothermal crystallization and subsequent melting of the 
polymer stepwise at intervals of a certain temperature or higher using a 
thermal analyzer. 
The present invention resides in a method for determining a branch 
distribution of a crystalline polymer which method comprises, using a 
thermal analyzer, melting the polymer at a temperature sufficiently higher 
than the melting point thereof, then cooling the melted polymer to room 
temperature in such a manner that isothermal crystallization of the 
polymer is allowed to take place stepwise at intervals of 3.degree. C. or 
higher at least from near the upper limit of the melting point of the 
polymer, then raising the temperature to melt the crystals and determining 
a branch distribution from the melt behavior.

DETAILED DESCRIPTION OF THE INVENTION 
The contents of the present invention will be described in detail 
hereinunder. 
Examples of the crystalline polymer whose branch distribution can be 
determined by the present invention include all polymers containing a 
crystalline portion, particularly polyolefins. Further, the method of the 
present invention is particularly effective for polyolefins of a structure 
having short chains on a straight-chain portion, e.g. LLDPE, because it is 
important in studying the relation of branch distribution to the physical 
properties of such polymers. 
The thermal analyzer used in the present invention used may be used a 
conventional differential scanning calorimeter (DSC) or differential 
thermal analyzer (DTA). It is desirable to use a thermal analyzer which is 
high in sensitivity and ensures a stable temperature control. From the 
standpoint of determination efficiency it is desirable to use a thermal 
analyzer which permits an automatic temperature control. 
In the determination method of the present invention, crystallizing a 
polymer continuously from near the melting point of the polymer over as 
long a time as possible according to the Flory's theory is considered most 
suitable accuracy because it is possible to determine branch densities in 
a continuous manner. On the other hand, however, at the time of melting it 
is necessary to detect the difference from a standard substance in view of 
the principle of the thermal analyzer, so it is necessary to maintain a 
rate of temperature increase at a certain value or higher. At the time of 
melting, therefore, there arises a delay of melting due to the phenomenon 
of superheating, thus resulting in a certain unavoidable deterioration of 
the resolving power. Having made studies about optimal conditions while 
taking both these inconsistent points into account, the present inventors 
found out that by allowing isothermal crystallization to take place 
stepwise at intervals of 3.degree. C. or more before determination of the 
melt behavior using a thermal analyzer there can be determined a branch 
distribution to a satisfactory extent despite of the superheating 
phenomenon during melting. If this interval is less than 3.degree. C., the 
separation of peaks on a melt behavior determination chart becomes 
incomplete and it is no longer possible to effect a highly accurate 
determination. Where a branch distribution is to be determined with high 
accuracy, it is preferable that the interval be 3.degree. C. or more and 
as close as possible to 3.degree. C. But this interval may be increased 
according to the object of the determination. Usually, there is used a 
certain interval in the range of 3.degree. to 10.degree. C. 
For determining a branch distribution from the melt behavior obtained it is 
necessary to determine a melting point of a sample whose branch density is 
known and in a narrow range by the method of the present invention and 
obtain a correlation between the melting point and the branch density to 
be used in the present invention. Since an isothermal crystallization 
temperature corresponding to the melting point can be obtained easily, 
there also can be easily obtained a correlation between such isothermal 
crystallization temperature and the branch density. Either of these 
correlations may be used in the analysis. 
Further, by measuring a melting heat value of a sample having a known 
branch distribution of a narrow range, it is possible to determine a 
crystallinity corresponding to each branch density on the basis of the 
measured value and a melting heat value (287.6 J/g) of a perfect crystal 
polyethylene. Then, by analyzing a melting heat chart obtained according 
to the method of the present invention using the crystallinity values thus 
obtained, it is possible to calculate the content of each section 
corresponding to each branch density. 
By practicing the present invention using the above method it is possible 
to determine a branch distribution extremely easily. 
As to the duration of each isothermal crystallization step, it is desirable 
to use a long time for each step to come as close as possible to an 
equilibrium state, but a duration of not longer than 200 minutes is 
suitable in consideration of shortening of the total time required for the 
determination. Further, the lower the temperature, the more rapidly the 
crystallization proceeds, so by shortening the duration of each stage 
successively with the shift from a high temperature stage to a lower 
temperature stage, it is possible to further shorten the determination 
time without great deterioration of accuracy. 
By practicing the present invention there can be obtained results almost 
equal to the results obtained by the TREF method with respect to the 
branch distribution of a crystalline polymer, in a much simpler manner and 
in a short time of several hours to 10 hours or so. Therefore, the method 
of the present invention can be employed as the quality control for a 
manufacturing process in a commercial plant. 
WORKING EXAMPLE AND COMATIVE EXAMPLE 
The present invention will be described below concretely in terms of 
working and comparative examples, but the invention is not limited 
thereto. 
EXAMPLE 
An ethylene-1-butene copolymer was divided into sections almost uniform in 
branch density by the TREF method and then each section was subjected to 
the determination by the DSC method according to the present invention to 
obtain a correlation of branch density and crystallization temperature. 
The branch density was determined using .sup.13 CNMR. The results are as 
shown in Table 1. 
TABLE 1 
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Isothermal 
Crystallization 
Temperature Branch Density 
(.degree.C.) (ethyl group/1,000 C) 
______________________________________ 
131 -- 
126 0-3 
121 3-7 
116 7-11 
111 11-16 
106 16-20 
101 20-24 
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Next, an ethylene-1- butene copolymer as a sample was melted sufficiently 
at 200.degree. C. and thereafter subjected to isothermal crystallization 
in accordance with the program shown in Table 2. 
TABLE 2 
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Set Temperature Isothermal 
Temperature 
Dropping Rate Crystallization 
(.degree.C.) 
(.degree.C./min) 
Time (min) 
______________________________________ 
200 
.dwnarw. 10 
136 0 
.dwnarw. 0.5 
131 120 
.dwnarw. 0.5 
126 90 
.dwnarw. 0.5 
121 80 
.dwnarw. 0.5 
116 70 
.dwnarw. 0.5 
111 50 
.dwnarw. 0.5 
106 30 
.dwnarw. 0.5 
101 20 
.dwnarw. 5 
20 15 
Total 475 
min 
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After completion of the isothermal crystallization, the copolymer was 
melted by heating at a rate of 5.degree. C./min up to 200.degree. C. and 
there was obtained a temperature-melting heat value relation. An 
isothermal crystallization temperature corresponding to each melting peak 
was obtained by comparing the melt behavior with the behavior of 
crystallization, and from the results obtained there were determined 
branch densities corresponding to the peaks, using the measured values 
shown in Table 1. 
Further, on the basis of the above results the content of each section 
corresponding to each branch density was determined according to the 
foregoing method. The results are set forth in Table 3 together with the 
results of the determination made on the same sample by the TREF method. 
The content of a section corresponding to a branch density of 0-24 ethyl 
group/1,000 C was assumed to be 100 wt %. 
TABLE 3 
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DSC Method According 
Branch Density 
to the Present TREF Method 
(ethyl group/1,000 C) 
Invention (wt %) 
(wt %) 
______________________________________ 
0-3 23 22 
3-7 29 27 
7-11 17 19 
11-16 12 13 
16-20 10 11 
20-24 9 8 
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As is seen from the above results, the time required for the isothermal 
crystallization is about 8 hours, which is not longer than one half of the 
time required in the conventional TREF method. In addition, the result of 
the determination of a branch distribution was in good coincidence with 
the result obtained by the TREF method.